you've made the hypothesis that the less rainfall a cactus plant gets, the taller it will grow, since cactuses like dry conditions.
what should be the independent variable in an experiment designed to test this hypothesis?
a. the height of the cactus plants
b. the hours of sunlight the cactus plants get
c. the amount of rain falling on the cactus plants
d. the type of soil the cactus plants are growing in