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 level after swimming pool sanitation \nSelect and explain. \nA. >0.5 mg/l, B. >0.7 mg/l, C. >1 mg/l, D. >0.7 ppm', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]