You've made the hypothesis that the less rainfall a cactus plant gets, the taller it will grow, since cactuses like dry conditions.
What is one thing that you should control in an experiment designed to test this hypothesis?
A. the height of the cactus plant
B. the hours of sunlight the cactus plants get
C. the number of cactus plants in the experiment
D. the amount of rain falling on the cactus plants