Which of the following is a physical change? a newspaper burns when placed in a fire. an iron chair rusts when left outside. a sample of water boils and releases gas. a plant changes carbon dioxide and water into sugar.

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.