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': 'Residual chlorine in chlorination of water should be: \nSelect and explain. \nA. 1 mg/L after 1 hr, B. 0.5 mg/L after 1 hr, C. 1 mg/L after 30 mins, D. 0.5 mg/L after 30 mins', 'role': 'user'}, {'content': "Answer: B \nExplanation: The minimum recommended concentration of free chlorine is 0.5 mg/L for one hour. The sum of the chlorine demand of the specific water plus the free residual chlorine of 0.5 mg/L constitutes the correct dose of chlorine to be applied. The chlorine has no effect on spores, protozoal cysts and helminthic ova, except in higher doses. Ref: Park's Textbook of Social and Preventive Medicine, 19th edition, Page 575.", 'role': 'assistant'}]