The Rate of Facilitated Diffusion of a Molecule Across a Membrane

The rate of facilitated diffusion of a molecule across a membrane will increase as the concentration gradient of the molecule across the membrane increases to a certain point. Eventually, an increase in the concentration of the molecule will not cause any further increase in facilitated diffusion. Thus, there is a maximal rate of facilitated diffusion. This is because

A) the diffusion constant depends on the concentration gradient.
B) the increased concentration gradient causes a situation far from equilibrium.
C) there are a limited number of carrier proteins in the membrane.
D) as the concentration gradient increases, molecules interfere with one another.
E) facilitated diffusion requires ATP energy.