LSAT Quiz: Strengthen

What this quiz covers

This quiz focuses on Strengthen, giving you a quick way to practice the rules, question types, and explanations that matter most for LSAT.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. The model relies on gradient-boosted decision trees trained on several years of historical churn data, which engineers periodically retune to reflect the platform's shifting catalog of shows and films.
2. In a randomized A/B test stratified by baseline risk, users assigned to algorithmic offers had lower churn than those assigned to generic offers, even after adjusting for exposure to the new hit series. (correct answer)
3. After the rollout, some longtime subscribers complained in support tickets that the volume of promotional notifications had increased noticeably, prompting a handful of them to disable alerts entirely on their accounts.
4. Industry-wide churn declined during the same quarter across most major streaming competitors, an effect analysts attribute largely to predictable seasonal factors rather than to any particular retention strategy.
5. The system performs poorly on very old devices that cannot display rich, image-based messages, so a small segment of subscribers received only plain-text versions of the tailored retention offers.

Explanation: The conclusion is that the algorithm itself reduces churn. The skeptics offer two rival explanations: offers went to users already likely to stay, and a new hit series boosted engagement. The best support neutralizes both. The randomized A/B test does exactly this: random assignment stratified by baseline risk rules out the targeting objection, and adjusting for exposure to the hit series rules out the show. That leaves the algorithm as the cause. The description of gradient-boosted trees and retuning explains how the model works but says nothing about whether it caused the drop rather than the rival factors. The complaints about extra notifications, if anything, describe a negative user reaction and give no evidence that offers reduced churn. The industry-wide seasonal decline actually supports a rival explanation—churn fell everywhere anyway—so it weakens rather than strengthens the causal claim. The point about old devices receiving plain-text offers concerns a technical limitation for a small segment and does nothing to eliminate the targeting or hit-series explanations. Only the randomized comparison isolates the algorithm's effect.

Question 2

1. The regional marketing campaign named Seabrook alongside three other coastal towns, all of which reported similar visitor gains that same weekend.
2. Hotels in Seabrook reported charging noticeably higher room prices during the festival weekend than during ordinary weekends earlier in the season.
3. A neighboring town without any rail link also saw a comparable rise in weekend visitors during the very same month as Seabrook's increase.
4. Traffic counts on highways into Seabrook did not increase, while ticketing records show that the majority of additional visitors arrived by train on the new line. (correct answer)
5. The rail operator offered discounted round-trip fares to Seabrook only on weekdays, when reported weekend visitor counts remained essentially unchanged.

Explanation: The argument concludes the rail line, rather than the marketing campaign or festival, caused the visitor increase. The winning choice shows that highway traffic did not grow while ticketing records place most of the extra visitors on the new train—this directly ties the rise to the rail line and rules out arrival by car. The choice about the campaign naming Seabrook and three other towns with similar gains actually weakens the argument, suggesting the campaign, not the rail, drove tourism. The hotels charging higher festival-weekend prices only shows demand during the festival, pointing to an alternative cause. The neighboring town without a rail link seeing a comparable rise undercuts the rail explanation by showing the increase happened even without rail service. The weekday-only discounted fares are irrelevant, since the counts that rose were weekend visitors, when those discounts did not apply. Only the traffic-and-ticketing evidence connects the specific mode of arrival to the rail line, so it alone strengthens the mayor's claim.

Question 3

1. Installing catalytic scrubbers of the type required will demand costly multi-week shutdowns at several of the region's older coal-fired plants during the transition.
2. Several coal-fired plants in the region already meet the new emissions standard and therefore will not need to install any additional scrubbing equipment.
3. Vehicle emissions across the region's busiest traffic corridors have increased slightly over each of the past two years, according to recent transit surveys.
4. In demographically and meteorologically similar regions that previously installed comparable scrubbers at plants, regional particulate levels fell substantially even though traffic and weather remained stable. (correct answer)
5. The regulation is scheduled to begin phased enforcement starting next spring, with full compliance by all affected plants required within roughly two years.

Explanation: The argument concludes that the scrubber rule will improve air quality despite the skeptics' point that traffic and weather also affect particulate levels. The claim that in demographically and meteorologically similar regions comparable scrubbers cut regional particulate levels substantially while traffic and weather stayed stable directly supports this: it isolates the scrubbers' effect from the very variables the skeptics raise, showing the plants' reductions were not overwhelmed. The option about costly multi-week shutdowns concerns expense, not whether air quality improves, so it is irrelevant to the conclusion. The option noting that several plants already meet the standard actually weakens the argument, since it means the rule produces fewer new reductions. The option about rising vehicle emissions supports the skeptics by suggesting traffic could offset the plants' gains. The option about the enforcement timeline tells us when the rule takes effect but nothing about whether it will deliver the promised air-quality benefit. Only the comparison-region evidence gives a reason to expect the reductions to hold up.

Question 4

1. Throughout the eight-week open-label trial, patients recorded their daily headache symptoms in diaries kept on their personal phones, which they submitted weekly to the study staff.
2. Many migraine sufferers, when starting a new prescription medication, also independently attempt dietary changes such as adjusting caffeine, sugar, or alcohol intake during the same period.
3. A separate open-label study, conducted with a chemically related compound and a comparable patient population, reported similarly reduced monthly migraine days after several weeks of treatment.
4. In a double-blind, randomized, placebo-controlled study with similar patients, those receiving Z had significantly fewer migraine days than those receiving placebo, and baseline severity and adherence were comparable across groups. (correct answer)
5. In some untreated patients monitored over comparable periods, migraine frequency tends to decrease naturally over time, even in the complete absence of any pharmacological intervention.

Explanation: The argument claims Z itself reduces migraine frequency, but the original trial can't rule out placebo effects, natural fluctuation, or uneven adherence. The choice describing a double-blind, randomized, placebo-controlled study directly answers all of these: if Z beats placebo when severity and adherence match, the improvement is genuinely attributable to Z. That's why it most strengthens the conclusion. The choice about patients recording symptoms in phone diaries only describes how data was collected—it adds nothing about whether Z, rather than expectation or natural change, caused the improvement. The choice noting that migraine sufferers often change their diets actually reinforces a confounder the passage already raises (caffeine adjustments), so it weakens rather than strengthens. The choice citing a related compound in another open-label study repeats the same uncontrolled design flaws, so it can't rescue the causal claim. The choice stating that migraine frequency naturally decreases over time supplies an alternative explanation for the reported improvement, undermining the argument. Only the controlled trial isolates Z's effect.

Question 5

1. Employees who slept more on weekends reported generally feeling better on Monday mornings, though their teams showed no measurable change in overall quarterly productivity scores.
2. Teams that skipped the workshop started a new project mix with tighter deadlines, which several managers believed could plausibly account for the observed differences in scores.
3. In a randomized trial the following quarter, employees assigned to the workshop outperformed otherwise similar employees who were not assigned, with no change in client demand. (correct answer)
4. Most surveyed employees said they would enthusiastically recommend the sleep-hygiene workshop to a friend, and a majority described the sessions as engaging and genuinely worthwhile.
5. The company cafeteria added several new decaf coffee options that quarter, and a few attendees mentioned appreciating the change during their afternoon breaks.

Explanation: The argument concludes the workshop itself improved productivity. The strongest support comes from the randomized trial: random assignment makes the workshop and non-workshop groups otherwise similar, so it neutralizes the selection worry that motivated people self-selected in, and holding client demand constant neutralizes the seasonal-lull worry. That directly isolates the workshop as the cause. The weekend-sleep finding is about how people feel, not about productivity, and even notes no change in scores, so it fails the ask. The claim that non-attending teams took on tighter deadlines introduces a confound and actually offers an alternative explanation for the score gap rather than ruling one out. Employees recommending the workshop or finding it engaging measures satisfaction, not whether productivity causally rose. The cafeteria's new decaf options are irrelevant to the workshop's effect and, if anything, add another possible cause. Only the randomized comparison rules out the rival explanations the skeptics raised.

Question 6

1. Anglers reported catching noticeably fewer fish during the final week before the ban took effect, a decline that local bait shops also confirmed in their own sales records.
2. The ban has proven unpopular with several local bait shops, many of which have reduced their operating hours and laid off part-time seasonal staff this year.
3. The survey volunteers received new nets and updated field training this year, changes that were introduced specifically to make their annual counts more thorough than before.
4. Water clarity on the lake improved markedly in early summer, due to an unusual absence of algae blooms that biologists attribute to the milder seasonal conditions.
5. Two nearby lakes experienced the same weather patterns and fertilizer regulations, but only Lake Alder adopted a fishing ban; fish counts rose only on Lake Alder. (correct answer)

Explanation: The correct answer sets up a controlled comparison: two nearby lakes shared the same weather and fertilizer conditions, yet counts rose only on the lake that added a fishing ban. By holding the critics' alternative causes constant and isolating the ban as the one difference, it directly supports the managers' causal claim. The report of fewer catches just before the ban says nothing about whether populations later rebounded, so it fails to strengthen. The bait shops' reduced hours and layoffs describe an economic side effect of the ban, not evidence about fish numbers. The new nets and improved training actually undercut the argument, since better sampling could explain the higher counts without any real rebound. Improved water clarity from an absence of algae blooms points to an alternative cause tied to the mild weather the critics raised, so it weakens rather than strengthens. Only the cross-lake comparison rules out competing explanations, making it the single best answer.

Question 7

1. Surveys of employees across every department found that most felt more trusted and reported higher morale in the months following the policy change.
2. An internal analysis matching teams on project complexity, size, and manager tenure found lateness dropped only in teams that adopted flexible schedules, even when staffing levels remained constant. (correct answer)
3. The firm substantially reduced its marketing and advertising budget during the same six-month period in which the flexible scheduling policy took effect.
4. Competitors that never adopted flexible schedules nonetheless reported comparable declines in late deliveries, reflecting a broad industry-wide improvement over the same period.
5. Some employees took advantage of the new flexibility to schedule longer midday breaks and to arrive later than they previously had.

Explanation: The conclusion is that flexible scheduling itself caused fewer late deliveries. The strongest support isolates flexibility from the rival explanations. The matched-analysis choice does exactly that: by comparing teams equal in complexity, size, and manager tenure, and holding staffing constant, it shows lateness fell only where flexibility was adopted — ruling out extra managers and shorter projects. The trust-and-morale survey is off target; feeling trusted is not evidence that flexibility, rather than staffing or project length, reduced lateness. The marketing-budget cut is irrelevant, since ad spending has no clear link to delivery timeliness. The competitor result actually weakens the conclusion: if firms without flexibility improved just as much, an industry-wide trend, not the policy, may explain Firm R's numbers. The longer-breaks detail describes how flexibility was used but gives no reason to think it improved on-time delivery. Only the matched comparison directly supports a causal role for flexibility.

Question 8

1. Pottery styles found at sites throughout the basin vary markedly in decoration and clay composition, suggesting distinct local traditions among the communities living there during that century.
2. Microwear and residue patterns indicate identical manufacturing techniques, and isotope analysis traces the chert to a single quarry not used elsewhere during that century. (correct answer)
3. Oral traditions recorded centuries later describe frequent travel, intermarriage, and ceremonial exchange of goods among the many river communities dispersed across the basin.
4. The blades are unusually well balanced and aesthetically pleasing, so their striking design could easily have been admired and adopted by widely separated and unrelated communities.
5. Nearby mountain sites, situated outside the river basin itself, contain a wholly different style of stone spearhead produced during a considerably earlier era.

Explanation: Strengthening questions ask which fact makes a conclusion more likely — not which one proves it. Here the conclusion is that a single dominant culture occupied the whole 300-mile basin, and the critics offer two rival explanations for the identical blades: trade or imitation. Choice B attacks one of those branches. If microwear and residue show identical manufacturing techniques, and isotope analysis ties every blade to a single quarry used nowhere else that century, then independent communities were not copying an admired design with their own local stone. The imitation story collapses. Be honest about the limits, though: one quarry and one technique are still consistent with a single workshop trading finished blades outward. B narrows the field rather than sealing the case — and on the LSAT, narrowing the field is exactly what "most support" means. No other option helps at all. A actively weakens the conclusion: varied pottery suggests distinct local traditions. C also weakens it by supplying the trade-and-exchange mechanism the critics wanted, among "many river communities." D restates the imitation objection in flattering language — attractive design invites independent adoption. E compares sites outside the basin from a much earlier era, so it is irrelevant on both geography and date. Takeaway: before choosing, ask whether an option supports, weakens, or is simply off-topic — and accept the strongest partial help when perfection is not on the menu.

Question 9

1. The exhibition budget exceeded that of any prior exhibition at the museum.
2. Surveys show museum members visit more often during summer months.
3. Entry surveys during the spike months indicated that a majority of visitors cited 'Ancient Oceans' as their primary reason for attending. (correct answer)
4. The festival drew particularly large crowds to downtown restaurants.
5. Other museums in the city also saw attendance increases during the festival.

Explanation: C directly links the increased attendance to the exhibition, reducing the likelihood that the city festival explains the spike. The other options are irrelevant or suggest competing causes.

Question 10

1. Students reported liking digital planners more than paper ones.
2. Late penalties were stricter that semester than in prior years.
3. Planner users also earned slightly higher grades overall than nonusers.
4. Before the study began, participants completed a conscientiousness inventory, and planner and non-planner groups did not differ on that measure. (correct answer)
5. Most participants were first-year students at the university.

Explanation: D addresses the main alternative explanation by showing the groups did not differ in conscientiousness at baseline. The other choices are irrelevant, confounding, or only correlate without isolating the causal effect.

Question 11

1. Several nearby suburbs that made no changes to their intersections experienced a similarly substantial decline in collisions during the same six-month period.
2. The collision decline occurred almost entirely at the twenty converted junctions, while collision rates at other city intersections did not change. (correct answer)
3. Average auto insurance premiums across the city fell slightly during the six-month period, mirroring the modest downward trend in reported collisions.
4. Roundabouts are considerably more expensive to build than stop-sign intersections, and the twenty conversions consumed most of the year's road-safety budget.
5. The public awareness campaign, which ran throughout the period, emphasized safe winter driving techniques for the city's most hazardous seasonal conditions.

Explanation: The officials must rule out the rival causes—the awareness campaign and improved seasonal visibility—that would have affected the whole city. If the drop in collisions happened almost entirely at the twenty converted junctions while other intersections stayed the same, the effect is tied specifically to the roundabouts, not to citywide factors. That is the strongest support. A similar decline in nearby suburbs that changed nothing actually points to a broader regional cause, weakening the roundabout explanation. Falling insurance premiums merely restate that collisions dropped; they add no evidence about the cause. The high cost of roundabouts and how much budget they used speaks to expense, not to whether they reduce collisions. The campaign's focus on winter driving strengthens a competing explanation—that the campaign or the season caused the drop—rather than supporting the roundabouts. Only the choice localizing the decline to the converted junctions isolates the roundabouts as the decisive factor.

Question 12

1. On residential streets similar to those with speed bumps but without them, accident rates did not decline during the same six-month period. (correct answer)
2. The cost of installing the speed bumps exceeded the town's initial budget estimates by roughly 15 percent, according to a public works audit.
3. Citywide, police officers issued substantially fewer speeding tickets across all neighborhoods in the several months following the speed-bump installations.
4. On one of the five speed-bump streets, a minor fender-bender occurred at an intersection whose collision had nothing to do with vehicle speed.
5. The speed bumps were painted bright reflective yellow and marked with warning signs so that approaching drivers could see them from a distance.

Explanation: The department must rule out the residents' rival explanation—that a citywide shift toward safer driving caused the drop. If comparable streets WITHOUT speed bumps saw no decline over the same period, then a general trend cannot account for the improvement, so the speed bumps become the likely cause. That directly strengthens the causal claim. The cost overrun tells you about budgeting, not about whether speed bumps reduced accidents, so it is irrelevant to the causal link. Fewer speeding tickets citywide actually supports the residents' rival explanation—it suggests driving improved everywhere, which would weaken the department's claim, not strengthen it. The single minor crash unrelated to speed is one anecdote that neither confirms nor undermines a 40 percent trend, and being speed-unrelated says nothing about the speed bumps' effect. The bright paint and signs describe a feature of the bumps but offer no evidence that accidents actually fell because of them; a plausible mechanism is not proof of an effect. Only the comparison with similar bump-free streets isolates the speed bumps as the cause, which is exactly what a strengthener must do here.

Question 13

1. Some university employees live in outlying suburbs beyond the reach of the region's convenient morning transit routes.
2. The university also raised its campus parking fees substantially during the previous summer to discourage solo commuting.
3. Transit ridership measured across the entire city rose slightly this winter compared with figures from the previous year.
4. Comparable neighborhoods without large employers offering free passes showed no decline in morning vehicle counts during the same period. (correct answer)
5. The free transit passes were funded by reallocating money that had previously supported the annual campus events budget.

Explanation: The administrators' worry is that snow, not the passes, caused the 15 percent drop. If snow were the cause, every area should show a decline. The winning choice tests exactly this: comparable neighborhoods that had no free-pass program showed no decline during the same snowy period. That rules out weather as the sole cause and points to the passes, strengthening the conclusion. The choice about employees living in outlying suburbs slightly weakens the argument, since it suggests some workers could not use the passes. The choice about raised parking fees the previous summer introduces a rival cause for reduced driving, which undercuts rather than supports the pass explanation. The choice about citywide ridership rising is too vague—a slight, general increase does not connect the campus-area drop to the passes and could equally reflect weather. The choice about how the passes were funded describes the budget source, which is irrelevant to whether the passes reduced traffic.

Question 14

1. Many participants had previously tried several other herbal supplements over the years without noticing any lasting reduction in their headaches.
2. Willowbark is derived from a bark long used in traditional folk medicine across several cultures to treat pain and inflammation.
3. In a double-blind randomized study of similar patients, those given Willowbark improved significantly more than those given a placebo. (correct answer)
4. Some participants reported mild nausea and dizziness during the first week, though these effects reportedly faded as the trial continued.
5. The study did not track participants' daily hydration levels or the amount of sleep they typically got each night.

Explanation: The conclusion is that Willowbark itself reduces headaches, but the critic points out that expectation, other habit changes, or continued over-the-counter remedies could explain the 60 percent result. The double-blind randomized placebo-controlled finding directly rules out those alternatives: patients who couldn't know what they took still improved more on Willowbark than on placebo, so the effect isn't just expectancy or habit change. That's why it best supports the conclusion. The claim that many participants had tried other supplements without success tells us nothing about whether Willowbark caused this improvement. That Willowbark comes from a bark used in folk medicine is irrelevant background; long use doesn't establish effectiveness. Reports of mild nausea and dizziness concern side effects, not whether headaches actually decreased. And noting that hydration and sleep went untracked, if anything, highlights an uncontrolled alternative explanation rather than strengthening the causal claim.

Question 15

1. Vitamin D is well established as essential for maintaining strong bones and a healthy immune response in adults.
2. Students in the study who had higher measured vitamin D levels also tended to exercise more frequently than their peers.
3. The group taking supplements reported noticeably fewer colds and sick days over the course of the academic semester.
4. Final exams administered this year were widely regarded as easier than usual across most academic departments at the college.
5. In a randomized, double-blind experiment at the college where study hours were equalized, supplemented students scored significantly higher than controls. (correct answer)

Explanation: The conclusion is causal: vitamin D itself, not merely healthier habits, improved exam performance. The randomized, double-blind study that equalized study hours directly rules out the studying-more objection and isolates vitamin D as the cause — exactly what strengthens a causal claim built on shaky observational data. The choice about vitamin D and strong bones and immunity is true but tangential; bone health says nothing about exam scores. The finding that high-vitamin-D students exercised more actually introduces a confound (exercise, not vitamin D) and undercuts rather than supports the conclusion. Fewer colds and sick days is a plausible health benefit but does not connect vitamin D to test performance, so it fails the actual ask. Exams being easier this year is irrelevant, because it would affect both groups equally and explains no gap between them. Only the controlled experiment removes the alternative explanations and supports the causal conclusion.

Question 16

1. During the three months after the bike lane opened, bicycle traffic on the avenue did not decrease compared to the three months before. (correct answer)
2. Many residents throughout the city support the construction of new bike lanes because they believe that regular bike commuting meaningfully reduces urban air pollution.
3. The city repaved the entire surface of the avenue only a few weeks before workers painted the new bike lane markings onto it.
4. Some accidents that involve bicycles on that avenue are minor enough that they are never formally reported to the city police.
5. Several other cities of comparable size have built bike lanes along their own major avenues at various points over the past few years.

Explanation: The council member treats the 40% drop in accidents as proof that the lane made cycling safer, then generalizes citywide. The key gap: fewer accidents could simply mean fewer cyclists were riding, not that each cyclist was safer. Confirming that bicycle traffic on the avenue did not decrease closes that gap—if just as many people rode yet accidents fell, the drop reflects genuine safety, supporting the conclusion. That is why it strengthens the argument. The repaving option is tempting, since smoother pavement could affect safety, but it offers an alternative cause for the drop rather than backing the claim that the lane itself made cycling safer—if anything, it weakens the argument. That some accidents go unreported concerns how the data was gathered; it neither supports the conclusion nor tells us whether the true reduction was larger or smaller. Residents' belief that bike commuting cuts air pollution addresses a benefit unrelated to accident rates, so it does nothing to support a safety claim. That other similar-sized cities have built bike lanes says nothing about whether those lanes reduced accidents, so it cannot strengthen the causal conclusion. Only stable ridership rules out the main competing explanation.

Question 17

1. Many of the businesses that received grants operated in the retail and food-service sectors, which historically account for a large share of Riverton's newly issued licenses each year.
2. The increase in licenses was concentrated among firms that met the grant eligibility criteria, while similar cities without grants and under the same state fee reduction saw no comparable rise. (correct answer)
3. The city promoted the grant program heavily through social media and local newspapers, generating far more public awareness of it than of the state's simultaneous reduction in filing fees.
4. Some qualified applicants were denied grants because the program ran out of funds, even though their proposed businesses met every published eligibility criterion the city had announced.
5. Riverton's population has grown steadily over the past decade, expanding the pool of residents who might plausibly seek a license to open a new business in the city.

Explanation: The argument concludes the grants caused the 30 percent rise, but the regional boom and the fee cut are alternative causes. The correct answer rules both out: growth clustered among grant-eligible firms, and comparable cities that had the fee cut but no grants saw no similar increase—so the grants, not the economy or fees, best explain Riverton's jump. The claim that grant recipients were mostly in retail and food service tells us who got grants, not whether grants caused new licenses. Heavy social-media promotion shows the program was visible, but visibility doesn't establish that it produced licenses rather than the economy or fees. Denying some qualified applicants for lack of funds actually suggests fewer grants than demand—it does nothing to link grants to the increase. Steady population growth over a decade supports the skeptics' 'expanding economy' explanation, weakening rather than strengthening the officials' case.

Question 18

1. The transit agency's social media following grew by 15 percent over the six-month period, and its online posts promoting the network reached a record number of local residents during those same months.
2. The previous year experienced a week-long bus driver strike that temporarily depressed ridership across most routes, an unusual disruption that made the comparison baseline lower than a typical year would have been.
3. Surveys of current passengers show that many riders value on-time reliability and route coverage more than ticket price when deciding whether to choose public transit over driving their own vehicles.
4. Fuel-price data and meteorological records show that gas prices were stable, weather typical, and no citywide events occurred that would boost transit use during those months. (correct answer)
5. The fare reduction disproportionately benefited low-income riders, a group that already rides buses far more frequently than higher-income residents who more often commute by private car.

Explanation: The officials argue that the cheaper fare caused the ridership jump, but skeptics list rival explanations: gas prices, weather, and special events. The correct choice directly closes off exactly those rival causes—confirming gas prices were stable, weather was typical, and no citywide events occurred—so the fare change is left as the best remaining explanation. The strike choice actually weakens the argument: it offers a competing reason ridership looks higher, since last year's baseline was artificially low. The social-media choice is tangential; more followers or online reach doesn't establish that the fare cut, rather than anything else, drove ridership. The reliability-survey choice reports rider preferences, not any change that occurred during the period, so it cannot explain a rise tied to the fare cut—and it even hints price matters less. The low-income choice explains who benefited but not whether the fare cut, versus gas prices, weather, or events, caused the overall increase. Only the choice ruling out the named alternatives strengthens the causal conclusion.

Question 19

1. In a double-blind randomized trial, participants assigned to drink chamomile before bed slept longer than participants assigned to a placebo beverage, with no differences in exercise or caffeine intake between groups. (correct answer)
2. In a follow-up interview, adults who disliked the taste of chamomile reported that they rarely drank it regularly, whether or not they had trouble falling asleep at night.
3. During the survey period, several tea companies funded a nationwide advertising campaign that heavily promoted chamomile as a natural aid for relaxation and calm.
4. Among survey respondents, adults who drank tea before bed also tended to read quietly in dim light for about 20 minutes beforehand, while non-drinkers did not.
5. The chamomile tea drinkers in the survey were considerably more likely to be female and to hold higher-paying office jobs than the non-tea drinkers were.

Explanation: The conclusion is causal: chamomile tea improves sleep duration. The strongest support comes from a controlled experiment. The double-blind randomized trial randomly assigns tea versus placebo and holds exercise and caffeine constant, so the longer sleep can be credited to the tea itself rather than to who chose to drink it. This directly answers the passage's worries about self-selection and unmeasured confounds. The choice about disliking chamomile's taste only explains who drinks it, not whether it affects sleep, so it does nothing for the causal claim. The choice about companies advertising chamomile speaks to why people might try it, not to whether it actually lengthens sleep. The choice about tea drinkers reading quietly beforehand introduces a rival explanation—the reading, not the tea, could be causing the extra sleep—so it weakens rather than supports the conclusion. The choice noting that tea drinkers were more often female and had higher-paying jobs highlights exactly the kind of group differences the passage warns about, suggesting a confound instead of a cause. Only the randomized, blinded trial rules out these alternatives and supports the conclusion.

Question 20

1. Surveys of commuters found that most bus routes throughout the surrounding region experienced noticeably fewer delays during the same year the parking ban took effect.
2. Merchants operating on a quiet residential side street several blocks away reported steady, essentially unchanged sales throughout the two quarters after the ban began.
3. The recently opened outlet mall sits just outside Riverton's city limits and advertises heavily to shoppers across the entire surrounding metropolitan region.
4. Many retailers along the main commercial street chose to extend their daily store hours in the months immediately after the curbside parking ban took effect.
5. Sales gains were concentrated among businesses on the previously parked-on blocks and were not observed on comparable blocks in neighboring towns that kept curbside parking. (correct answer)

Explanation: The officials claim the parking ban itself caused the sales jump, but the skeptics offer rivals: a growing regional economy and a new outlet mall. To strengthen the argument, you want evidence that isolates the ban from those region-wide factors. The correct choice does exactly this: gains showed up specifically on the affected blocks but NOT on similar blocks in nearby towns that kept curbside parking. That comparison rules out broad regional growth and points the cause back to the policy and its location. Why the others fail: fewer bus delays across the whole region actually supports the skeptics' regional explanation and never ties the gain to Riverton's street. Stable sales on a distant residential side street tell you nothing about the commercial street where the ban applied. The outlet mall detail strengthens a competing cause, since a regionally advertised mall could itself explain the higher receipts. Extended store hours introduce a different possible cause for the sales rise, weakening the claim that removing parking was responsible. Only the keyed choice provides a controlled comparison that connects the increase uniquely to the policy.