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': 'Level of residual chlorine after one hour \nSelect and explain. \nA. l mg/L, B. 1.5 mg/L, C. 0.5 mg/L, D. 2 mg/L', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]