Why is genetic variation important from an evolutionary standpoint?

• A. If all organisms were the same, the entire population would be vulnerable to particular pathogens, like viruses.
• B. All evolutionary adaptations (e.g. the origin of forelimbs) are the result of the gradual build up of genetic differences between organisms over geologic time.
• C. Evolution (at the population level) refers to changes in the frequencies of genes in the population over time.
• D. All of the above.