Which of the following best compares the effects of genetic drift and natural selection on the genetic makeup of a population?

a.) they are both selective processes that decrease the genetic variation of a population.

b.) they are both selective processes, but genetic drift increases genetic variation and natural selection does not.

c.) they both decrease genetic variation, but genetic drift is random and natural selection is not.

d.) they both increase the genetic variation of a population, but genetic drift is directed and natural selection is not.