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': '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: Water to be chlorinated should not be turbid as it impedes the efficiency of chlorination. Chlorine demand for water should be estimated and free residual chlorine of 0.5mg/L should be present against subsequent water contamination. A contact period of ONE HOUR at least is essential for free residual chlorine to kill bacteria and viruses. No effect on spores, protozoal cysts, helminthic ova except in higher doses (PARK TB 25th edition pg 775)', 'role': 'assistant'}]