Which of the following BEST describes the theory of natural selection?

a The genetic make-up of a population gradually changes from one generation to the next.
b Living organisms change over time.
c Each species produces many more offspring than can survive long enough to reproduce. The offspring that do survive contain genes that are inherited from the parents.
d There is variation among the members of a species. Some of this variation is genetically determined and so can be passed on to future generations.