The correct answer is D. All the above.
Hydrology, dissolved oxygen in water, and temperature are all factors that contribute to the purification of sewage in rivers.
Hydrology refers to the movement of water on, above, and below the Earth’s surface. The flow of water in rivers helps to flush out sewage and other pollutants.
Dissolved oxygen is essential for the survival of aquatic organisms, including bacteria that help to break down sewage. The amount of dissolved oxygen in water can be affected by temperature, rainfall, and other factors.
Temperature also affects the rate at which sewage is broken down. In general, higher temperatures lead to faster decomposition.
Therefore, all of the above factors play a role in the purification of sewage in rivers.