Final answer:
Facilitated diffusion differs from simple diffusion in that it requires carrier or channel proteins to help substances cross the cell membrane, while both processes move substances down their concentration gradients.
Step-by-step explanation:
The difference between diffusion and facilitated diffusion lies in the mechanisms they use to transport substances across the cell membrane. Diffusion is a process where molecules spread from an area of higher concentration to an area of lower concentration without any energy expenditure. In contrast, facilitated diffusion also moves substances down their concentration gradient, but it requires the presence of carrier proteins or channel proteins to help these molecules cross the cell membrane. Therefore, the correct answer to the question is C. Facilitated diffusion uses carrier proteins to move substances in or out of the cell.