Why Would Dairy Farming Result in the Spread of a Gene

Why would dairy farming result in the spread of a gene allowing adults to digest lactose, when prior to that, only infants could?

A) Being able to get nutrition from cow's milk as an adult would keep individuals better fed and therefore healthier and able to produce more children who would then also have that trait.
B) Keeping cows would tend to trigger any mutations related to being able to drink milk since that would be make individuals healthier and able to produce more children who would have the same trait.
C) Farming is a harder way of life, so to survive, an individual would need to be able to digest milk and so would gradually gain that ability.
D) Being able to get nutrition from cow's milk as an adult would keep individuals better fed and therefore healthier and better able to feed their children.
E) Prior to this, hunter gatherer societies didn't keep cows, so there was no reason to adapt to drinking milk.