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: \nSelect and explain. \nA. Sand bed, B. Zoological layer, C. Supernatan, D. Under drainage layer', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Zoological layer', 'role': 'assistant'}]