Which of the Following Events Occurs After the Formation of Disulfide

Which of the following events occurs after the formation of disulfide bonds during the conversion of preproinsulin to mature insulin?

A) folding of proinsulin into a stable conformation
B) transport of the molecule through the membranes
C) cleavage of the peptide sequence that joins the A and B chains
D) cleavage of the leader sequence
E) posttranslational modification of specific proline residues