Insulin Cannot Be Taken Orally. Why

Insulin cannot be taken orally. Why?

A) Insulin is a protein and would be broken down by the human digestive tract if taken orally.
B) Oral insulin has been shown to lead to the development of severe peptic ulcers.
C) Oral insulin does not control blood glucose levels in a consistent manner and is therefore very unreliable.
D) Oral insulin takes too long to work in the body and is therefore ineffective in lowering blood glucose levels.