ACT Science Quiz: Experimental Design

20 questions · exam conditions

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Experimental DesignQuestion 1 of 20

A lab group investigated whether a caffeine solution increases heart rate in Daphnia (water fleas). Hypothesis: higher caffeine concentration increases Daphnia heart rate. Procedure: (1) Prepare caffeine solutions at 0.0% (control), 0.1%, and 0.5% (mass/volume) in pond water. (2) Place one Daphnia in a depression slide with 1 mL of solution and allow 2 minutes to acclimate. (3) Under a microscope, count heartbeats for 15 seconds and multiply by 4 to estimate beats/min. (4) Repeat for 10 Daphnia per concentration. (5) The same student performed all counts to reduce observer variation. The group compared mean beats/min among concentrations.

A flaw in the experimental design is that:

Heartbeats were counted for 15 seconds instead of a full minute.

Different Daphnia were used at each concentration, preventing within-subject comparisons.

Water temperature was not controlled or recorded during microscope observations.

Using one observer eliminates bias, so additional observers were required.

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ACT Science Quiz

ACT Science Quiz: Experimental Design

Practice Experimental Design in ACT Science with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Experimental Design, giving you a quick way to practice the rules, question types, and explanations that matter most for ACT Science.

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.

All questions

Question 1

A flaw in the experimental design is that:

Heartbeats were counted for 15 seconds instead of a full minute.
Different Daphnia were used at each concentration, preventing within-subject comparisons.
Water temperature was not controlled or recorded during microscope observations. (correct answer)
Using one observer eliminates bias, so additional observers were required.

Explanation: The experiment tests whether caffeine concentration affects Daphnia heart rate by comparing different caffeine solutions. A critical flaw is that water temperature was not controlled or recorded during microscope observations, yet temperature is known to significantly affect heart rate in ectothermic organisms like Daphnia. The microscope light could heat the slide, and different observation times could lead to different temperatures, introducing an uncontrolled variable that could mask or exaggerate caffeine effects. Using different Daphnia (B) is actually appropriate for this design, and counting for 15 seconds (A) is a reasonable time-saving measure that doesn't constitute a flaw.

Question 2

A chemistry class tested whether the surface area of calcium carbonate affects reaction rate with hydrochloric acid. Hypothesis: powdered CaCO $_3$ reacts faster than a single large chunk of the same mass. Procedure: (1) Measure 2.00 g of CaCO $_3$ as either powder or one intact chip. (2) Add 50.0 mL of 1.0 M HCl to a 250 mL flask. (3) Add CaCO $_3$ , immediately seal the flask with a stopper connected to a gas syringe, and start a timer. (4) Record CO $_2$ volume in the syringe every 10 s for 120 s. (5) Repeat 3 trials per CaCO $_3$ form, using fresh acid each time. The class compared CO $_2$ volume vs. time curves.

Which hypothesis is being tested?

Higher HCl concentration produces more CO $_2$ than lower HCl concentration for the same CaCO $_3$ .
Powdered CaCO $_3$ produces CO $_2$ more rapidly than a single chip of equal mass. (correct answer)
CO $_2$ gas dissolves in HCl less when the flask is sealed with a stopper.
CaCO $_3$ powder has a greater total mass than a CaCO $_3$ chip measured on a balance.

Explanation: The experiment's stated hypothesis is that powdered CaCO₃ reacts faster than a single large chunk of the same mass, focusing on how surface area affects reaction rate. The procedure tests this by comparing CO₂ production over time between powder and chip forms of equal mass (2.00 g). The hypothesis being tested is that powdered CaCO₃ produces CO₂ more rapidly than a single chip of equal mass, as increased surface area provides more contact points for the acid reaction. The experiment does not test different HCl concentrations (A), gas solubility (C), or mass measurement accuracy (D).

Question 3

A student examined whether the angle of an inclined plane affects the acceleration of a rolling cart. Hypothesis: increasing ramp angle increases cart acceleration. Procedure: (1) Set a 1.00 m track on a lab bench and raise one end to create angles of 5°, 10°, and 15° (measured with a protractor). (2) Place a motion sensor at the bottom and align it with the cart's path. (3) Release the cart from rest at the same marked starting line for each angle, without pushing. (4) Record velocity vs. time for 2.0 s and compute acceleration from the slope of the best-fit line. (5) Perform 5 trials per angle.

The experiment could be improved by:

Using a heavier cart at higher angles to prevent it from accelerating too quickly.
Measuring and controlling wheel friction or using the same cart and wheels throughout. (correct answer)
Releasing the cart from different starting positions to sample more velocities.
Replacing the motion sensor with a stopwatch to avoid electronic measurement errors.

Explanation: The experiment tests how ramp angle affects cart acceleration by measuring acceleration at different angles while keeping other factors constant. The experiment could be improved by measuring and controlling wheel friction or using the same cart and wheels throughout, as friction is a significant factor affecting acceleration that could vary between trials or equipment. This would ensure that observed differences in acceleration are due to angle changes rather than friction variations. Using a heavier cart (A) would change the system being studied, different starting positions (C) would violate the controlled conditions, and replacing the motion sensor (D) would likely reduce measurement precision.

Question 4

A researcher tested whether adding a commercial enzyme cleaner reduces protein-based stain mass on cotton fabric. Hypothesis: fabric treated with enzyme cleaner will lose more stain mass than fabric treated with water alone. Procedure: (1) Cut 24 identical 5 cm × 5 cm cotton squares and label them. (2) Apply 0.50 mL of egg-white solution to each square and dry for 2 hours. (3) Measure and record each square's mass using a balance (±0.001 g). (4) Randomly assign squares to 3 groups (n=8): Enzyme Cleaner, Water Only, and No Wash. (5) For Enzyme Cleaner, soak in 100 mL cleaner diluted 1:10 for 10 min with gentle stirring; for Water Only, soak in 100 mL distilled water for 10 min; for No Wash, store dry for 10 min. (6) Rinse washed groups for 30 s under running distilled water, then air-dry all squares for 12 hours. (7) Re-measure mass and compute mass change for each square.

Which hypothesis is being tested?

Egg-white stains increase fabric mass more on cotton than on synthetic fibers.
Rinsing under distilled water removes more stain than rinsing under tap water.
Enzyme cleaner causes a greater decrease in stained-fabric mass than water alone. (correct answer)
Air-drying for 12 hours removes a constant amount of water from all fabrics.

Explanation: The experiment's stated hypothesis is that fabric treated with enzyme cleaner will lose more stain mass than fabric treated with water alone. The procedure tests this by comparing mass changes in three groups: Enzyme Cleaner, Water Only, and No Wash control. The hypothesis being tested is that enzyme cleaner causes a greater decrease in stained-fabric mass than water alone, as this directly compares the effectiveness of the enzyme treatment versus water treatment. The other options describe different hypotheses about fabric types (A), water types (B), or drying effects (D) that are not being tested in this experiment.

Question 5

A researcher tested whether activated charcoal removes dye from water. Materials: 6 clear cups, blue dye solution, activated charcoal powder, coffee filters, and a balance. Procedure: (1) Add 100 mL of the same dye solution to each cup. (2) Add charcoal to reach 0.0 g, 0.5 g, or 1.0 g (2 cups per amount). (3) Stir each cup for exactly 30 s. (4) Filter each mixture through a coffee filter into a clean cup. (5) Measure the filtered liquid's color intensity using a phone app that reports a "blue value" from 0 (no blue) to 255 (very blue). Lower blue values indicate more dye removal.

Step 4 (filtering) was included in order to:

Remove charcoal particles so the color measurement reflects dissolved dye, not suspended black powder. (correct answer)
Increase dye concentration by evaporating water during filtration, making differences easier to detect.
Chemically convert blue dye into a colorless form by exposing it to oxygen in the filter.
Standardize the amount of charcoal added, because filters absorb excess charcoal to a fixed level.

Explanation: The purpose of filtering is to remove the charcoal particles from the solution before measuring color intensity. The experiment tests whether charcoal removes dye by adsorption, but the presence of black charcoal powder in the liquid would interfere with accurate color measurement. Filtering separates the solid charcoal (which may have adsorbed dye) from the liquid phase, allowing the app to measure only the dissolved dye remaining in solution. Answer A correctly identifies this purpose - removing charcoal particles so color measurement reflects dissolved dye concentration. The filter doesn't concentrate dye or chemically convert it.

Question 6

A chemistry student tested whether activated charcoal removes dye from water. Materials: 6 flasks, 100 mL blue dye solution (same initial absorbance), activated charcoal, filter paper, spectrophotometer. Procedure: (1) Add 0 g, 0.5 g, or 1.0 g charcoal to flasks (2 flasks per amount). (2) Swirl for 60 s and let sit for 5 min. (3) Filter each mixture to remove charcoal particles. (4) Measure absorbance of the filtrate at the dye's peak wavelength. Expected outcome: higher charcoal mass yields lower absorbance. Step 3 was included in order to:

Ensure the dye fully dissolves before measurement, preventing undissolved dye crystals from forming.
Remove charcoal solids that would scatter light and interfere with spectrophotometer absorbance readings. (correct answer)
Allow charcoal to chemically react with the dye to form a new colored compound in solution.
Increase dye concentration by evaporating water so absorbance readings are easier to compare.

Explanation: Step 3 was included to remove charcoal solids that would scatter light and interfere with spectrophotometer absorbance readings. Activated charcoal particles would cause light scattering and create turbidity that affects absorbance measurements independently of dye concentration, so filtration ensures accurate measurement of dissolved dye remaining in solution. Choice B correctly identifies this analytical purpose: removing particles prevents optical interference. Choice C incorrectly suggests chemical reaction, when charcoal physically adsorbs dye molecules.

Question 7

A scientist is testing the hypothesis that a new drug reduces blood pressure. Participants are given the drug, and their blood pressure is measured before and after. A flaw in the experimental design is that:

Participants are not randomly selected.
Blood pressure is not measured frequently.
The dosage of the drug is consistent.
There is no control group. (correct answer)

Explanation: The hypothesis is that the new drug reduces blood pressure in participants. However, the experimental design lacks a crucial control group - participants who receive a placebo or no treatment - which is necessary to determine if observed changes are due to the drug rather than other factors like natural variation or placebo effects. Without a control group, any blood pressure changes cannot be confidently attributed to the drug's effectiveness. The absence of a control group is a fundamental flaw that prevents drawing valid conclusions about the drug's efficacy.

Question 8

Students tested whether an antacid tablet neutralizes stomach acid faster when crushed. Materials: 6 beakers, 100 mL of 0.10 M HCl per beaker, antacid tablets (same brand), mortar and pestle, pH probe, stirrer. Procedure: (1) In 3 beakers, add one whole tablet; in 3 beakers, add one tablet crushed to powder. (2) Stir all beakers at 300 rpm. (3) Record pH every 15 s for 5 min. (4) Define "neutralized" as pH reaching 4.0. Results: crushed tablets reached pH 4.0 in 75 s; whole tablets in 180 s. The primary purpose of this experiment was to:

Determine whether crushed tablets contain more base than whole tablets of the same brand.
Test whether increasing tablet surface area increases the rate of acid neutralization. (correct answer)
Measure the exact molarity of HCl using antacid tablets as a primary standard.
Compare how stirring speed changes pH over time during an acid-base reaction.

Explanation: The primary purpose is to test whether increasing tablet surface area increases the rate of acid neutralization. The experiment compares whole tablets (low surface area) against crushed tablets (high surface area) while measuring how quickly pH changes during neutralization, directly testing surface area's effect on reaction rate. Choice B correctly identifies this surface area hypothesis. Choice A incorrectly suggests comparing base content, when the tablets are identical except for physical form.

Question 9

Students tested whether a UV-blocking window film reduces fading of colored paper. Materials: 10 identical colored paper strips, 2 windows receiving similar sunlight, UV-blocking film, clear tape, color chart or phone colorimeter app. Procedure: (1) Tape 5 strips to the inside of Window F covered with UV-blocking film. (2) Tape 5 strips to Window N with no film. (3) After 14 days, measure each strip's color intensity using the same phone app and lighting conditions. Results: Window F strips retained higher intensity than Window N strips. The experiment could be improved by:

Replacing sunlight with darkness so fading is prevented and differences are easier to detect.
Using different phone apps for each window to ensure measurements are independent.
Measuring fading only by visual inspection, because apps introduce too much numerical error.
Placing both sets of strips on the same window, with half behind film and half uncovered. (correct answer)

Explanation: The experiment could be improved by placing both sets of strips on the same window, with half behind film and half uncovered. This would ensure identical light exposure conditions while isolating the UV-blocking film's effect, eliminating confounding variables like different window orientations, sun angles, or light intensities. Choice D correctly identifies this improvement: same-window placement controls for environmental variables. Choice B incorrectly suggests using different apps, which would introduce measurement inconsistency rather than improve control.

Question 10

Researchers examined whether running shoes with "energy return" foam reduce oxygen consumption at a fixed speed. Materials: treadmill, metabolic cart, 8 runners, Shoe A (standard foam), Shoe B (energy-return foam). Procedure: (1) Each runner completed two 10-min runs at 10 km/h on different days. (2) Order was randomized: half wore A first, half wore B first. (3) For each run, oxygen consumption ( $\mathrm{VO_2}$ ) was recorded during minutes 8–10 and averaged. (4) Runners were instructed to eat the same breakfast before each session. Mean $\mathrm{VO_2}$ : Shoe A = 38.2 mL/kg/min; Shoe B = 37.0 mL/kg/min. Which hypothesis is being tested?

Randomizing shoe order eliminates all physiological differences between runners during treadmill exercise.
Energy-return foam decreases oxygen consumption at a fixed speed relative to standard-foam shoes. (correct answer)
Runners who eat identical breakfasts will have identical oxygen consumption across all treadmill speeds.
Energy-return foam increases running speed at constant oxygen consumption compared with standard foam.

Explanation: The hypothesis being tested is that energy-return foam decreases oxygen consumption at a fixed speed relative to standard-foam shoes. The experiment holds running speed constant (10 km/h) while comparing oxygen consumption between the two shoe types, directly testing whether the energy-return technology reduces metabolic cost. Choice B correctly identifies this hypothesis: lower oxygen consumption at fixed speed indicates improved efficiency. Choice D incorrectly suggests speed varies, when the protocol specifically maintains constant speed.

Question 11

A scientist tested whether adding yeast increases CO $_2$ production in bread dough. Materials: flour, water, salt, sugar, yeast, 6 sealed bottles with balloons, balance. Procedure: (1) Prepare two dough mixtures: Mix Y contains yeast; Mix N is identical but without yeast. (2) Place 50.0 g of each mixture into 3 bottles each. (3) Stretch a balloon over each bottle opening to capture gas. (4) After 60 min at 30°C, measure balloon circumference as an estimate of gas produced. Results: Mix Y balloons were larger than Mix N. The primary purpose of this experiment was to:

Compare how bottle size affects fermentation rate by changing the volume available for gas expansion.
Test whether salt is required for balloon inflation by preventing gas from escaping the bottle.
Measure the exact mass of CO $_2$ produced using balloon circumference as a direct mass measurement.
Determine whether yeast produces CO $_2$ gas during dough fermentation compared with an otherwise identical mixture. (correct answer)

Explanation: The primary purpose of this experiment was to determine whether yeast produces CO₂ gas during dough fermentation compared with an otherwise identical mixture. The experiment uses yeast presence/absence as the variable while measuring gas production through balloon inflation, directly testing yeast's role in CO₂ generation during fermentation. Choice D correctly identifies this hypothesis: comparing gas production between yeast and no-yeast conditions. Choice B incorrectly focuses on salt's role, when salt is present in both mixtures as a controlled variable.

Question 12

A group tested whether a smartphone's battery drains faster at low temperature. Materials: 6 identical phones, freezer (0°C), room (22°C), battery logging app. Procedure: (1) Charge all phones to 100% and set screen brightness to 50%. (2) Place 3 phones in the freezer and 3 at room temperature. (3) Play the same 30-min video on loop with Wi‑Fi on for 3 hours. (4) Record battery percentage every 30 min. Result: freezer phones dropped to 40% after 3 h; room phones to 65%. A flaw in the experimental design is that:

Using identical phones prevents generalizing to other brands, so the hypothesis cannot be tested.
Wi‑Fi was on, so battery drain cannot be measured at all under any temperature condition.
The freezer may reduce Wi‑Fi signal strength, changing power use independently of temperature effects. (correct answer)
Recording battery percentage every 30 minutes provides too many data points for meaningful comparison.

Explanation: The experimental design flaw is that the freezer may reduce Wi-Fi signal strength, changing power use independently of temperature effects. Wi-Fi radios consume more power when signals are weak as they increase transmission power to maintain connections, creating a confounding variable where both temperature and signal strength affect battery drain simultaneously. Choice C correctly identifies this confounding variable: signal strength changes could alter power consumption regardless of temperature. The freezer environment likely attenuates Wi-Fi signals compared to room temperature conditions.

Question 13

A teacher tested whether rinsing rice reduces starch in the cooking water. Materials: 6 identical pots, 300 g uncooked white rice per pot, water, sieve, timer, colorimeter, iodine solution. Procedure: (1) For 3 pots, rinse rice under running water for 60 s, then drain. For 3 pots, do not rinse. (2) Add 600 mL water to each pot and boil for 12 min with lids on. (3) Collect 10 mL cooking water from each pot and add 2 mL iodine solution. (4) Measure absorbance at 620 nm; higher absorbance indicates more starch. Results: rinsed mean absorbance = 0.35; not rinsed = 0.62. The primary purpose of Step 3 was to:

Convert starch into sugar so the colorimeter can measure sweetness of the cooking water.
Create a colored complex with starch so starch amount can be estimated by absorbance. (correct answer)
Neutralize acids in the cooking water so boiling time is identical for all pots.
Sterilize the cooking water sample to prevent microbial growth during colorimeter readings.

Explanation: The primary purpose of Step 3 was to create a colored complex with starch so starch amount can be estimated by absorbance. Iodine forms a characteristic blue-black complex with starch, and the intensity of this color (measured as absorbance) directly correlates with starch concentration in the cooking water. Choice B correctly identifies this analytical purpose: iodine creates a measurable colored indicator of starch presence. Choice A incorrectly suggests converting starch to sugar, when iodine actually forms a detection complex.

Question 14

Engineers tested whether a new biodegradable mulch film reduces soil water loss. Materials: 12 identical pots, potting soil, mulch film, plastic wrap, scale (±0.1 g), water. Procedure: (1) Add 500 g dry soil to each pot and water each with 200 g water. (2) Cover 6 pots with mulch film; cover 6 pots with plastic wrap (control cover). (3) Poke 6 identical holes (5 mm) in every cover to allow gas exchange. (4) Place all pots under the same lamp for 48 h. (5) Measure mass loss of each pot (soil + cover) after 48 h; assume mass loss is water evaporated. Mean mass loss: mulch = 62 g; plastic wrap = 40 g. A flaw in the experimental design is that:

Poking holes in both covers ensures gas exchange, which invalidates evaporation measurements.
Mass loss measures only soil erosion, not evaporation, so results cannot relate to water loss.
The plastic wrap control blocks water loss differently than bare soil, confounding mulch effectiveness. (correct answer)
Using identical soil mass prevents detecting evaporation differences between cover types.

Explanation: The experimental design flaw is that plastic wrap creates a different water loss mechanism than bare soil, confounding the comparison with mulch effectiveness. The experiment should compare mulch film against bare soil (no cover) to isolate mulch's effect on evaporation, but instead compares two different cover types that both affect water loss differently than natural soil conditions. Choice C correctly identifies this confounding variable: plastic wrap blocks water loss through a different mechanism than mulch, making it impossible to determine mulch's true effectiveness. The control should represent the natural condition being improved upon.

Question 15

A physics class tested whether a parachute's canopy area affects fall time. Materials: 9 identical toy figures (same mass), plastic bags, string, scissors, meter stick, stopwatch, 3 m drop height. Procedure: (1) Build parachutes with canopy areas 100 cm $^2$ , 200 cm $^2$ , and 400 cm $^2$ (3 parachutes each). (2) Attach each parachute to a toy with equal string length. (3) Drop each toy from 3 m indoors and time from release to floor contact. (4) Repeat each drop twice and average times. Results: larger canopy area produced longer fall times. What is the independent variable in this experiment?

The number of repeated drops, because repetition changes the final average time.
The canopy area of the parachute, changed across three designed size conditions. (correct answer)
The drop height, because it determines the distance the toy falls before landing.
The average time it takes the toy to reach the floor after being dropped.

Explanation: The independent variable is the canopy area of the parachute, changed across three designed size conditions. This is the factor deliberately manipulated by the researchers (100, 200, and 400 cm²) to test its effect on fall time while other variables like drop height, toy mass, and string length remain constant. Choice B correctly identifies canopy area as the independent variable. Choice D describes the dependent variable (fall time), while choice C describes a controlled variable.

Question 16

In an experiment to see if sugar affects yeast fermentation, yeast cultures were grown with 5%, 10%, and 15% sugar concentrations. The amount of carbon dioxide produced was measured. The dependent variable is:

Fermentation temperature
Amount of carbon dioxide produced (correct answer)
Type of yeast used
Sugar concentration in the cultures

Explanation: The experiment tests whether sugar concentration affects yeast fermentation by varying sugar levels (5%, 10%, 15%) and measuring the fermentation output. Fermentation produces carbon dioxide as yeast breaks down sugar, so CO2 production directly indicates fermentation activity. The dependent variable is the amount of carbon dioxide produced because this is the measured outcome that responds to changes in sugar concentration. Sugar concentration is the independent variable being manipulated, while CO2 production is the response being measured to assess fermentation rates.

Question 17

A researcher designs an experiment to test the effect of sound on sleep quality by exposing subjects to different noise levels. What is the independent variable?

Duration of sleep
Sleep quality
Number of subjects
Noise level (correct answer)

Explanation: The hypothesis is that sound affects sleep quality in exposed subjects. The experimental design systematically varies noise levels (quiet, moderate, loud) as the factor being manipulated by the researcher, making noise level the independent variable. Sleep quality would be measured as the dependent variable (the response), while factors like duration of sleep and number of subjects would be controlled. The independent variable is the factor that researchers deliberately control and change to test its effects.

Question 18

A biologist conducted an experiment to test the effect of different fertilizer types on crop yield. Three types of fertilizers were used on three separate fields, and the crop yield was measured after 3 months. Which hypothesis is being tested?

Fertilizers decrease the growth rate of crops.
All fertilizers increase crop yield equally.
Crop yield is unaffected by fertilizer type.
Different fertilizers affect the crop yield differently. (correct answer)

Explanation: The experiment tests whether different fertilizer types produce different effects on crop yield. By applying three different fertilizers to separate fields and measuring yields, the researcher can compare the effectiveness of each treatment. The hypothesis being tested is that different fertilizers affect crop yield differently, as this captures the purpose of comparing multiple fertilizer types. Options B and C suggest no difference or uniform effects, which would not require testing multiple fertilizer types.

Question 19

Students conducted an experiment to test the effect of pH on enzyme activity by measuring reaction rates at pH 4, 7, and 9. Which hypothesis is being tested?

Reaction rate is independent of pH.
Enzyme activity is consistent across pH levels.
pH affects the temperature of the solution.
Enzyme activity varies with pH levels. (correct answer)

Explanation: The students deliberately changed one thing, pH, testing it at 4, 7, and 9, and measured reaction rate as the outcome. A design like that is built to reveal whether the enzyme's activity rises or falls as acidity changes, so the hypothesis under test is that enzyme activity varies with pH levels. The statement that reaction rate is independent of pH describes the null result the experiment might return, not the prediction the experiment was set up to examine, and saying enzyme activity is consistent across pH levels amounts to the same thing. The idea that pH affects the temperature of the solution names an outcome the students never measured, since their measurement is reaction rate. To identify the hypothesis, pair the variable that was intentionally changed with the variable that was measured.

Question 20

Researchers investigated whether a disinfectant reduces bacterial growth on countertops. Materials: 15 identical sterile tiles, a nonpathogenic bacterial culture, sterile swabs, nutrient agar plates, and two spray bottles labeled Solution A and Solution B. Procedure: (1) Add the same volume of bacterial culture to each tile and spread evenly. (2) After 5 min, spray tiles: 5 tiles with Solution A, 5 tiles with Solution B, and 5 tiles with sterile water. (3) Wait 10 min. (4) Swab each tile using the same swabbing pattern and pressure, then streak each swab onto an agar plate. (5) Incubate plates at 37°C for 24 hr and count colonies (CFUs). The team concluded Solution A was best because its plates had the fewest colonies.

The experiment could be improved by:

Removing the water-sprayed tiles, because controls reduce the ability to detect disinfectant differences.
Randomly assigning tiles to treatments and blinding the colony counter to reduce bias and confounding. (correct answer)
Using different bacterial species on each tile to ensure results generalize to all bacteria.
Incubating plates at different temperatures for each solution to match each disinfectant's optimal conditions.

Explanation: The experiment could be improved by randomly assigning tiles to treatments and blinding the person counting colonies. Random assignment ensures that any systematic differences in tile position, bacterial application, or other factors are distributed equally across treatment groups rather than biasing one treatment. Blinding the colony counter prevents unconscious bias in counting, where knowing which treatment was applied might influence the count. Answer B correctly identifies these improvements - randomization and blinding - as ways to reduce bias and confounding. The control group (water-sprayed tiles) is essential for comparison, not something to remove.