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: Slow sand filter occupies large area (more space).\nSlow and filter requires less skilled operation.\nSlow sand filter removes more bacteria and has better bacterial control (99.9-99.99%)\nSlow sand filter requires longer duration for purification.\nSize of sand is smaller (0.2-0.3 mm).\nSlow sand filter requires more time (duration).', 'role': 'assistant'}]