Diphtheria Toxin Is a Potent Inhibitor of Protein Synthesis

Diphtheria toxin is a potent inhibitor of protein synthesis. Which one of the following statements closely describes its mode of action?

A) It inhibits the binding of N-formyl methionine-tRNA to subunit of ribosomes.
B) It inhibits the peptidyl transferase activity of 50S subunit of ribosomes.
C) It catalyzes the transfer of adenosine diphosphate ribosyl moiety of NAD to the elongation factor EF-2.
D) It causes premature chain termination by covalent attachment of the toxin molecule to the growing polypeptide chain.
E) none of the above