Which of these is a case of selective breeding?

a. to increase the production of milk in a herd of milk cows, the highest producing cows were chosen as the mothers.

b. over time, more milk cows at the diary farm began to produce larger amounts of milk.

c. a gene was identified that increases milk production. this gene was added to calves in the womb.

d. the genome of a group of cows was analyzed the gene for milk production.