Which of the following provides the best example of a change in genetic
traits over several generations in response to natural selection?

a) Farmers choose to plant only seeds from those crops that proved to be resistant to
insects.

b) The number of spots on a group of giraffes varies from one individual to the next

c) A group of chimpanzees learns to communicate with their trainer to obtain food.

d) The average root depth of one species of plant increases as annual rainfall in the area goes down.