Crossing a membrane by simple diffusion can be distinguished from facilitated diffusion because:

A.	Simple diffusion does not require energy: facilitated diffusion requires a source of ATP.
B.	Simple diffusion can only move material in the direction of a concentration gradient; facilitated diffusion moves materials with and against a concentration gradient.
C.	Simple diffusion is not saturable; facilitated diffusion rates are limited by the number of functional membrane proteins and can be saturated
D.	Simple diffusion is found only in prokaryotes while facilitated diffusion is found only in eukaryotes.