Bacterial aerobic oxidation of polluted water in biological oxidation ponds is done to purify it. Presence of bacteria helps in

• A. coagulation and flocculation of colloids.
• B. oxidation of carbonaceous matter to CO2.
• C. nitrification or oxidation of ammonia derived from breakdown of nitrogeneous organic matter to the nitrite and eventually to the nitrate.
• D. all (a), (b) and (c).