Which of the following does not affect rate of diffusion?

The magnitude of the diffusing tendency from one region to another is directly propoionate to the cross-sectional area across which diffusion is taking place and the concentration gradient, or chemical gradient, which is the difference in concentration of the diffusing substance divided by the thickness of the boundary (Fick's law of diffusion). Ref:Ganong's Review of Medical Physiology 23rd edition, Chapter 1.