21. Cell membranes are asymmetrical. Which of the following statements are the most likely explanation for the membrane's asymmetrical nature? (A) Since the cell membrane forms a border between one cell and another in tightly packed tissues such as epithelium, the membrane must be asymmetrical. (B) Since cell membranes communicate signals from one cell to another, the cell membranes must be asymmetrical. (C) The two sides of a cell membrane are structurally different, and different phospholipids are found in each side of the membrane. (D) Proteins only function on the cytoplasmic side of the cell membrane, which results in the membrane's asymmetrical nature.