Which of the following statements about the sodium-potassium pump is true:
a. it transports hydrogen ions out of the cell
b. it transports 3 sodium ions out of the cell in exchange for 2 potassium ions
c. it transports 2 sodium ions out of the cell in excahnge for 2 potassium ions
d. it transports 2 sodium ions out of the cell in exchange for 3 potassium ions
e. it transports water directly out of the cell