18.在冷天時植物細胞通常藉著下列何種變化來維持其細胞膜的流動性？ (A) activating a proton pump (B) increasing the number of phospholipids with saturated hydrocarbon tails (C) increasing the proportion of peripheral proteins (D)increasing the number of phospholipids with unsaturated hydrocarbon tails (E) increasing the concentration of cholesterol in the membrane