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
Minimum chlorine content of water after chlorination should be ? 
Select and explain. 
A. 0.5mg\/L, B. 5 mg\/L, C. 0.05mg\/L, D. 50mg\/L
A: 0.5mg\/L
Answer: A: 0.5mg\/L 
Explanation: 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 .