This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
[{'content': 'True about sand filter is \nSelect and explain. \nA. Occupies less space, B. More expensive, C. Requires longer duration, D. Sand size 0.4-0.7mm', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Slow sand filters occupy more space, requires long duration require less skilled operators and gives good bacterial quality when compared to rapid sand filters. But it takes more time for the operation. Rate of filtration is 200 m.g.a.d in the rapid sand filter but only 2-3m.g.a.d. in the slow sand filter (PARK TB 25th edition pg 773)', 'role': 'assistant'}]