The correct answer is C. Both thermal pollution and artificial eutrophication are a result of power plants and other industrial activity. Thermal pollution occurs when power plants and other industries release heated water into bodies of water, which can lead to a decrease in oxygen levels and disruption of aquatic ecosystems. Artificial eutrophication occurs when pollutants from power plants and other industries enter bodies of water, which can cause an increase in nutrients and lead to large mats of algae blooming. Both of these processes are difficult to identify and control, as the sources of the pollutants are often hard to trace.