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': 'Vital layer in slow sand filter is seen \nSelect and explain. \nA. On the sand bed, B. Top of water, C. Near filter valves, D. None', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]