The sodium-potassium pump regulates the amount of sodium and potassium inside of cells, particularly nerve and muscle cells. In this process, an ATP is burned, causing three sodium ions to be pumped out and exchanged for two potassium ions, which are pumped in by a membrane protein. Both ions go against their concentration gradients. The sodium-potassium pump is an example of

a. phagocytosis.
b. passive transport.
c. active transport.
d. facilitated diffusion.