Which is a controlled variable in this experiment?
a. number of pots
b. amount of water
c. amount of phosphate
d. amount of blooms on a plant

4

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.