D! The Cell membrane in found in all cells, and it is a single (mono) layer.
The difference in an area with high concentration and an area with low concentration is called concentration gradient.
<h3>What is concentration gradient?</h3>
Concentration gradient refers to the difference between the concentrations of two solutes in a solution.
A solution usually contains an area or region with higher concentration and an area with lower concentration.
The difference between these two areas of different concentrations is referred to as a concentration gradient.
Learn more about concentration gradient at: brainly.com/question/13050811
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They are important because so people do not get sickness