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': 'The minimum contact period after chlorination: \nSelect and explain. \nA. 1/2 hour, B. 1 hour, C. 2 hours, D. 3 hours', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans: B (I hour) Ref: Park's Textbook of Preventive and Social Medicine,20th edition, page 625Explanation:Refer to the above explanation.", 'role': 'assistant'}]