How to find experimental design in biology Practice Test

The chart above shows the height growth of three different plant species after a period of 2 weeks. Each plant species was grown in 4 different soil mediums. All the plants were grown in the same environment with equal amounts of light, water, and oxygen.

Based on the chart, soil Medium C has what effect on plant growth?

Mitochondria make 90% of the energy needed by the body to sustain life. The Mitochondrial Free Radical Theory of Aging (MFRTA) theorizes that individuals who live longest produce fewer mitochondrial oxygen reactive species than individuals that have a shorter life span. Therefore, lifespan will increase if fewer mtROS are produced, and lifespan will decrease if more mtROS are produced. An experiment was done to test this theory, and the results are shown in the chart below. Four test groups of flies were involved, two groups consisted of females, and two groups consisted of males.

| | Test group 1 | Test group 2 | Test group 3 | Test group 4 | | | ------------------- | ---------------- | ---------------- | ---------------- | ------- | | # of mtROS | 3.9 | 2.5 | 3.2 | 2.7 | | Lifespan | 110 days | 120 days | 95 days | 89 days |

What, if added, would strengthen the results of the experiment?