What factors play a role in the rate at which molecules like o₂ and co₂ can diffuse?
a. the distance between the two areas of concentration of the molecule
b. the magnitude of the concentration gradient of the molecule
c. the size of the molecule that is diffusing
d. the physical nature of the media through which the molecule is diffusing

The correct answer is b the magnitude of the concentration gradient of the molecule.

Explanation:

Dffusion is one of most important membrane transport process that allow several molecules such as gases oxygen and carbon dioxide to pass directly across the plasma membrane.The is no involvement of  carrier proteins or ion channels during passive diffusion.

    During passive diffusion molecules are transported along their concentration gradient that means from the region of high concentration to region of low concentration,until the concentration of both regions become same.