Why Are Humans That Are Not Lactose Intolerant Still Able

Why are humans that are not lactose intolerant still able to digest milk?

A) because of the mutation in a single cytosine to thymine in the regulatory region of the lactase allele
B) because of the mutation in a single thymine to cytosine in the regulatory region of the lactase allele
C) because of the absence of the lactase allele
D) because humans are mammals and all mammals digest milk all their lives