Why Is Insulin Only Given by Injection and Not as an Oral

Why is insulin only given by injection and not as an oral drug?

A) Injected insulin works faster than oral drugs to lower blood glucose levels.
B) Insulin is a small protein that is destroyed by stomach acids and intestinal enzymes.
C) Insulin is a "high-alert drug" and could more easily be abused if it were available as an oral agent.
D) Oral insulin has a high "first pass loss" rate in the liver and would require very high dosages to be effective.