After blood becomes oxygenated, after blood becomes oxygenated, it does not return to the heart, but goes directly to capillaries that supply the body's cells with oxygen. it returns to the heart, and is then pumped to body cells. it returns to the heart, and is then pumped to the lungs. it does not return to the heart, but goes to the nose and mouth. it does not return to the heart, but goes directly to the lungs.

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.