Why did 40% of the human population evolve to tolerate lactose?

Question

A. Within cultures that rely on milk-producing animals, individuals who tolerated lactose had a survival advantage
B. Babies rely on milk to survive, so they need to be able to tolerate lactose
C. Lactose tolerance allows people to enjoy a wide variety of food, including milk, butter, ice cream, and cheese
D. People who have an intolerance physical pain, so a tolerance evolved to avoid that

Answer 1

A. Within cultures that rely on milk-producing animals, individuals who tolerated lactose had a survival advantage