18. Which of the following is true regarding membrane function? (A) a caveola contains a very high concentration of cholesterol and unsaturated phospholipids. (B) scaffolding proteins, particularly those containing BAR domains, result in curvature of the cell membrane. (C) lateral membrane diffusion within a biological membrane is a completely unrestricted process (D) membrane fusion with vesicles is often accomplished with the aid of a SNARE protein. (E) both b and d are correct.