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: Social & Preventive Medicine
[{'content': 'Following are true regarding chlorination except ? \nSelect and explain. \nA. Residual chlorine is 0.5 mg/lit., B. Contact period - 30 minute, C. Water should not be turbid, D. Chlorine demand should be estimated', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. is 'b' i.e., Contact period - 30 minute Contact period of 1 hour is recommended.", 'role': 'assistant'}]