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
In slow sand filters, the percentage of waters that is bacteria-free: 
Select and explain. 
A. 100%, B. 99.90%, C. 99%, D. 90%
B: 99.90%
Answer: B: 99.90% 
Explanation: The total bacterial count is reduced by 99.9 to 99.99%.
Essentials of preventive and community dentistry 
Soben Peter 
5th edition