At the mouth of the Mississippi River, large concentrations of nitrogen and phosphorus flow into the Gulf of Mexico, causing nutrient overloading, algal blooms, and extremely low levels of oxygen. Which revolutions are most responsible for this problem

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.