In the human body oxygen is absorbed by the lungs and nutrients are absorbed by the small intestine in a single-cell organism this absorption directly involves

In a single cell organism, the absorption of nutrients and control of certain things to come inside or go outside the cell directly involves the cell membrane. Although, the basic function of the cell membrane is to protect the cell from its surroundings. It also facilitates the transport of substances needed for survival.

d. air pressure decreases as altitude increases.