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': 'Effective size of sand in rapid sand filter is \nSelect and explain. \nA. 0.2 mm, B. 0.1 mm, C. 0.8 mm, D. 0.5 mm', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]