There is much evidence that supports how life has changed over time due to variation in the gene pool of different species. Which of the following choices below represents a scientific theory that explains these changes?

A- As environments and ecosystems change over time, the gene pool also changes because organisms that have the best suited traits are more likely to survive and reproduce.

B- Because the environments and ecosystems show little change over time, organisms are already adapted for surviving in their given habitats for which they are made.

C- As environments and ecosystems change over time, the gene pool also changes because organisms must adapt to survive and those that acquire suitable traits are more likely to survive and reproduce.

D- Because the environments and ecosystem show little change of time, the organisms that choose to adapt to different environments will migrate and become suited for new habitats.