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