This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
Trickling filter is used in – 
Select and explain. 
A. Primary treatment of sewage, B. Scondary treatment of sewage, C. Oxygen ponds, D. Sewage farming
B: Scondary treatment of sewage
Answer: B: Scondary treatment of sewage 
Explanation: None