21. Which one of the following statements is incorrect? (A) Rafts are enriched in sphingolipids and cholesterols. (B) A region with more than 20 residues of high hydropathy index is presumed to be a transmembrane segment. (C) The positively charged Lys, His, and Arg residues of membrane proteins occur more commonly on the cytoplasmic face of membranes. (D) Glycosyl phosphatidylinositol (GPI)-linked proteins are always on the inner face of the plasma membrane. (E) Lamin A uses the farnesyl group to anchor at the nuclear inner membrane. Unable to cleave pre-Lamin A/C from the nuclear inner membrane may cause premature aging.