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': 'The minimum recommended level of residual chlorine in the drinking water is for one hour \nSelect and explain. \nA. 0.5 mg/L, B. 0.25 mg/L, C. 1.0 mg/L, D. 2.0 mg/L', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]