This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Environment and health, Subject name: Social & Preventive Medicine
[{'content': 'Minimum recommended concentration of free chlorine drinking water at the end of 1 hour is - \nSelect and explain. \nA. 0.5 mg/L, B. 0.1 mg/L, C. 1.0 mg/L, D. 1.5 mg/L', 'role': 'user'}, {'content': "Answer: A \nExplanation: The minimum recommended level of residual chlorine is 0.5 mg/L. Park's Textbook of Preventive and Social Medicine, 25th edition, Page No. 774", 'role': 'assistant'}]