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': 'The permissible levels of E.coli (number/ml) in potable water is \nSelect and explain. \nA. 20, B. 15, C. 10, D. 0', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]