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': 'An inexpensive and efficient method of sewage disposal for a small community is – \nSelect and explain. \nA. River outfall, B. Oxidation pond, C. Trickling filter, D. Activated sludge', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]