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
[{'content': 'Trickling filter is used in – \nSelect and explain. \nA. Primary treatment of sewage, B. Scondary treatment of sewage, C. Oxygen ponds, D. Sewage farming', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]