Scientists investigated the effect of one-sided light on the distribution of the plant hormone auxin in seedling shoot tips. They grew seedlings in uniform conditions, cut off the shoot tips, and placed them on agar blocks as shown in the diagram below. 
State two control variables that scientists must keep constant in this investigation to ensure valid results.
A scientist proposed the following hypothesis: "One-sided light causes auxin to migrate laterally from the illuminated side of the shoot tip to the shaded side." Explain how the evidence from Setup C and Setup D supports this hypothesis.
Another scientist hypothesized: "One-sided light destroys or breaks down auxin on the illuminated side of the shoot." Explain how the data from the experiment show that this hypothesis is incorrect.