Saturation occurs when

A) molecules are moved by the use of vesicles.
B) the energy required to move molecules results from a high-energy bond.
C) a group of carrier proteins is operating at its maximum rate.
D) a preference of a carrier protein for a substance is demonstrated based on the differing affinities of the carrier for the substrates.
E) a carrier molecule has the ability to transport only one molecule or a group of closely related molecules.