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: Dental
[{'content': 'In slow sand filters, the percentage of waters that is bacteria-free: \nSelect and explain. \nA. 100%, B. 99.90%, C. 99%, D. 90%', 'role': 'user'}, {'content': 'Answer: B \nExplanation: The total bacterial count is reduced by 99.9 to 99.99%.\nEssentials of preventive and community dentistry\xa0\nSoben Peter\xa0\n5th edition', 'role': 'assistant'}]