move through the lipid bilayer of a plasma membrane most rapidly

A. CO2

Explanation:

The structure of the lipid bilayer allows small, uncharged substances such as oxygen and carbon dioxide, and hydrophobic molecules such as lipids, to pass through the cell membrane, down their concentration gradient, by simple diffusion.

For instance, when the fresh oxygen molecules in your lungs come into contact with your red blood cells, they diffuse rapidly across your red blood cell membranes into the cells, or down their concentration gradient.

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