Which of the Following Statements Is Correct for Protein Structure

Which of the following statements is correct for protein structure?

A) The a helix is stabilized primarily by ionic interactions between the side chains of amino acids.
B) Disulfide bonds are common in cytoplasmic proteins but are not present in most proteins of the extracellular matrix.
C) The stability of the quaternary structure in proteins is a result of covalent bonds between the subunits.
D) The heat denaturation of proteins is in most cases reversible by slow cooling.
E) Glycine and proline do not usually participate in a-helical structures.