49. Which one of the following statements about membranes is true? (A) Most plasma membranes contain more than 70% proteins. (B) Sterol lipids are common in bacterial plasma membranes. (C) Sterol lipids are common in human cell plasma membranes. (D) Sterol lipids are common in plant cell plasma membranes. (E) The plasma membranes of all cell types within a particular organism have basically the same lipid and protein composition.