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
The permissible levels of E.coli (number/ml) in potable water is 
Select and explain. 
A. 20, B. 15, C. 10, D. 0
D: 0
Answer: D: 0 
Explanation: None