This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
[{'content': 'Minimum chlorine content of water after chlorination should be ? \nSelect and explain. \nA. 0.5mg\\/L, B. 5 mg\\/L, C. 0.05mg\\/L, D. 50mg\\/L', 'role': 'user'}, {'content': 'Answer: A \nExplanation: park\'s textbook of preventive medicine 23rd edition *principle of chlorination is to ensure that a "free"residual chlorine of 0.5 mg/litre at end of one hour contact .', 'role': 'assistant'}]