Which of the following is NOT an example of natural selection?

a. Plants with thorns are less likely to be eaten by herbivores than other members of the same species that lack thorns.
b. Bacterial populations in hospitals develop resistance to drugs used to combat infection by them.
c. Scientists breed cows that give greater amounts of milk than their ancestors.
d. Fruit fly larvae with an enzyme to break down alcohol are better able to feed on fermenting fruit than those that lack the enzyme.
e. Female fish that produce more eggs leave more offspring than those that produce fewer eggs.