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 vital layer in a slow sand filter is ? \nSelect and explain. \nA. Sand bed, B. Under drainage, C. Zoological layer, D. Supernatant', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]