Geralda is listing all of the conditions under which natural selection occurs. So far, she has listed:

1. The environment changes.

2. Organisms in a population have different traits.

Which other condition should she include in her list?
a. All organisms in the population adapt and reproduce.
b. The environment returns to normal after some time.
c. The environment can support only new organisms.
d. Only some of the organisms survive and reproduce.