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 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: Option A - wrong, slow sand filter occupy more space Option B - wrong, it is less expensive Option C - correct, rate of filtration is very slow (2-3m.g.a.d.) hence takes longer time Option D - wrong, effective sand size in slow sand filter is 0.2-0.3 mm (PARK TB 25th edition pg 773)', 'role': 'assistant'}]