Answer:
A. depend on a concentration gradient
Explanation:
Simple and facilitated diffusion are the two methods of transport of substances down their concentration gradient. In both processes, substances are moved from the region of their higher concentration to that of their lower concentration. Since simple diffusion and facilitated diffusion are passive processes, there is no requirement of energy input to drive them. Both are driven by the concentration gradient of the substances only.
I think the answer is A
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