Why do cells need to be able to control movement of substances across the plasma membrane?

To move substances against a concentration or electrochemical gradient, the cell must use energy. Small substances constantly pass through plasma membranes. Active transport maintains concentrations of ions and other substances needed by living cells in the face of these passive movements.

Explanation:

true, i don't really know what you're asking but from the info you provided im guessing the answer is true.

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.