This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Water, Subject name: Social & Preventive Medicine
[{'content': 'Percentage available chlorine from bleaching powder is:- \nSelect and explain. \nA. 10%, B. 33%, C. 66%, D. 95%', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Bleaching powder contains : 33% available chlorine. Bleaching powder required for disinfection of 1000 litres of water = 2.5 grams. Also Remember, Free/ residual chlorine levels recommended in:- Drinking water = >= 0.5 mg/ L ; with contact period of 1hr. Drinking water to kill cyclops = >= 2.0 mg/L ; with contact period of 1hr. Swimming pools of India =>= 1.0 mg/L ; with contact period of 1hr.', 'role': 'assistant'}]