Human babies have a gene that produces an enzyme that breaks down the sugar found in milk. Over time, this gene is shut off in most adult humans. A mutation in this gene allows some adults to continue to drink milk without discomfort. Why would this mutation have allowed early humans to survive?

A- it provided milk free from bacteria

B- it provided vitamins that could only be produced in milk

C- it provided more cattle that could be raised by early humans.

D- it provided an alternative source of nourishment for early humans.