Lactose Is Milk Sugar That Is Broken Down by the Enzyme

Lactose is milk sugar that is broken down by the enzyme lactase. The reason that some people are "lactose intolerant" could be because

A) the enzyme lactase only works in the laboratory, not in the human body.
B) the pH of the digestive system does not allow lactase to function properly.
C) the temperature of the digestive system does not allow lactase to function properly.
D) they drink too much milk and have inactivated the lactase in their digestive system.
E) they are missing the enzyme lactase in their digestive system.