[Answer] When blood supplies oxygen and food to a cell surface and these molecules diffuse across we might expect some “resting time.” However if we inspected most cell membranes in a living human body we would find a nearly continuous flow of substances back-and-forth across the plasma membrane. Which of the following is NOT a reason why?A) Cell metabolism is constantly using up oxygen and raw materials; this lowers their concentration so more will diffuse into the cell.B) Cell metabolism is constantly producing waste molecules; this increases their concentration so more will diffuse out of the cell.C) Cells produce useful secretions; this increases their concentration and more will diffuse out of the cell.D) Cell metabolism constantly produces carbon dioxide in respiration; this increases its concentration so more will diffuse out of the cell.E) Cells are living entities and in order to live can never “rest”.