Which of the following is the least likely to increase the rate of diffusion? A. Small concentration gradient. B. Higher temperature. C. Higher concentration of molecules. D. Small molecule size

The rate of diffusion of a substance depends on four factors which are: temperature, concentration gradients, diffusion distance and the diffusion and host material. The rate of diffusion is affected by differences in concentration gradient; the higher the difference in concentration gradient, the higher the diffusion rate. Thus, small concentration gradient will not increase the rate of diffusion. The correct option is A.