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': "In an epidemic of plague, within 48 hours of application of insecticides by spraying the 'X' cheopis index should fall to - \nSelect and explain. \nA. 1, B. 0, C. 2, D. 5", 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]