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': 'The sewage ground water is purified by: \nSelect and explain. \nA. Oxidation pond, B. Soakage pit, C. Activated sludge process, D. Any of the above', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Activated sludge process', 'role': 'assistant'}]