Why are proton pumps in root-hair plasma membranes important?a. They pump protons into cells, generating a membrane potential (voltage).b. They allow toxins to be concentrated in vacuoles, so the toxins do not poison enzymes in the cytoplasm.c. They set up an electrochemical gradient that makes it possible for roots to absorb cations and anions.d. They set up the membrane voltage required for action potentials to occur.