Question

When domestic sewage mixes with river water

• A. Small animals like rats will die after drinking river water
• B. The increased microbial activity releases micro nutrients such as iron
• C. The increased microbial activity uses up dissolved oxygen
• D. The river water is still suitable for drinking as impurities are only about 0.1%

Answer 1

The correct option is C The increased microbial activity uses up dissolved oxygen

Any mixing of sewage with river water increases BOD and decreases DO due to the decomposing activity of microbes.