Cows have been producing milk on the same farm for over 100 years. The cows have experienced the same conditions on the farm over time, but milk production has almost doubled. Which of these best explains the increase in the cows' milk production over time? a. The farmers only bred cows that gave the most milk.

b. Cows that gave more milk were more likely to have smaller offspring.

c. The farmers only bred cows that already had offspring.

d. Cows that gave more milk were more likely to attract mates.