Final answer:
The Clean Water Act of 1972 has led to a decrease in nitrate and phosphate from point sources but not from nonpoint sources, which still pose challenges to water quality due to their diffuse nature. Therefore correct option is C
Step-by-step explanation:
Since the Clean Water Act of 1972, the quantities of nitrate and phosphate entering surface waters have decreased for point sources but not for nonpoint sources. The Clean Water Act helped to regulate point sources of pollution, such as pipes from factories or sewage treatment plants, through permits and standards.
On the other hand, nonpoint source pollution, which includes runoff from agricultural fields and urban areas, remains a challenge. Nonpoint source pollution is more diffuse and harder to control, continuing to contribute to eutrophication and harmful algal blooms in aquatic ecosystems. Measures like banning phosphate-containing detergents and promoting agriculture practices that reduce runoff can help mitigate some of these issues.