What Makes Some People "Lactose Intolerant" as Adults

What makes some people "lactose intolerant" as adults?

A) Some people don't produce any enzymes, including the one needed to digest lactose.
B) Lactose can only be digested by people who produce enough lactic acid to digest it.
C) Lactose-intolerant people don't produce adequate amounts of lactase.
D) The active site of the lactose-digesting enzyme can vary from person to person.