The ocean is divided into distinct zones of marine life. Which three factors are used to define the zones? A. available sunlight, distance from shore, and water depth B. water temperature, salinity, and water depth C. sea-floor spreading, volcanic activity, and sedimentation rate D. available sunlight, salinity, and oxygen content of the water

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.