Which of the following is a post-translational modification of a polypeptide?
a. Removal of introns and splicing of exons
b. Formation of a polysome that allows simultaneous formation of many polypeptides from one mRNA transcript
c. The growing polypeptide signals the ribosome to attach to the ER
d. Complementary base pairing of mRNA and tRNA in the ribosome
e. Cleavage of a polypeptide into two or more chains