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': 'In a slow sand filter, the element responsible for yielding bacteria-free water is the - \nSelect and explain. \nA. Valve, B. Vital layer, C. Supernatant water, D. Under-drainage system', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Vital layer", 'role': 'assistant'}]