The correct answer is: C. In sewer treatment plants.
Aerobic activity is the process of breaking down organic matter using oxygen. This process is essential for the proper functioning of sewer treatment plants, as it helps to remove pollutants from the wastewater. Aerobic activity is most efficient in the presence of high levels of oxygen, which is why sewer treatment plants are designed to provide aeration.
In freshly produced sewage, there is a high concentration of organic matter. This organic matter provides food for the bacteria that break it down. However, the concentration of oxygen in freshly produced sewage is low. This is because the sewage has not had time to mix with air. As a result, aerobic activity is low in freshly produced sewage.
At sewer pipes, the concentration of oxygen is even lower than in freshly produced sewage. This is because the sewage is flowing through the pipes and does not have time to mix with air. As a result, aerobic activity is very low at sewer pipes.
In sewer treatment plants, the concentration of oxygen is high. This is because the sewage is mixed with air in the aeration tanks. As a result, aerobic activity is high in sewer treatment plants.
Therefore, the correct answer is: C. In sewer treatment plants.