Please help me I really need it.

Aboy jumps into a cold swimming pool and his body tempature goes down hus muscles ,blood vessels

Which of the following statements is true about the relationship between genetic variation and natural selection? a. genetic variation must be present in a population before natural selection can act on it. b. genetic variation arises in a population as a result of natural selection. c. natural selection can act on a population whether there is genetic variation within the population or not. d. after natural selection acts on a population, the amount of genetic variation in the population always increases.

In general, characteristics that an organism survive and reproduce become more common in a population over time. what mechanism of evolution cause this change?