Which is the function of the cholesterol molecules in a cell membrane

This helps slightly immobilize the outer surface of the membrane and make it less soluble to very small water-soluble molecules that could otherwise pass through more easily. Without cholesterol, cell membranes would be too fluid, not firm enough, and too permeable to some molecules. (www.cholesterol-and-health.com/Cholesterol-Cell-Membrane.html)

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if you listed your answers right then i believe your answer is b.