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': 'True about slow sand filter is \nSelect and explain. \nA. Occupies less space, B. More expensive, C. Requires longer duration, D. Sand size 0.4-0.7 mm', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]