31. Which of the following statements regarding transmembrane proteins is correct?
 (1) typicaily exposed only to one side of the membrane.
(2) released from the membrane by a gentle extraction procedure such as sait treatment.
(3) often further attached to the membrane via a GPI anchor.
(4) sometimes covalently attached to a fatty acid chain that inserts into the membrane.
(5) cannot contain B sheets in the part of their structure that interacts with the membrane interior.
(A) (1) and (2)
(B)(3) only
(C) (4) only
(D) (3) and (4)
(E) (4) and (5)