21. Which one of the following situations is a result of human activities? a) decay of leaves in a forest adds to soil fertility b) natural selection on an island changes gene frequencies c) acid rain in an area kills fish in a lake d) ecological succession following volcanic activity reestablishes an ecosystem

The correct answer is c) acid rain in an area kills fish in a lake.<br /><br />Acid rain is a result of human activities, specifically the emission of sulfur dioxide (SO2) and nitrogen oxides (NOx) into the atmosphere from industrial processes, power plants, and vehicles. These pollutants react with water vapor in the atmosphere to form sulfuric acid and nitric acid, which then fall to the ground as acid rain. Acid rain can have harmful effects on aquatic ecosystems, such as killing fish in lakes, as mentioned in the question.