Why must insulin be injected intravenously, while other enzymes such as ... ...

Why must insulin be injected intravenously, while other enzymes such as lactase can be taken orally, by pill.
(A) Insulin is a protein; if it was taken by mouth, it would be broken down into its constituent amino acids, and would never enter the blood.
(B) ...
(C) ...
(D) ...
(E) ...

*This question is included in 16. The Digestive, Respiratory, and Urinary Systems: Absorption and Excretion