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 usual incubation period of pertusis is - \nSelect and explain. \nA. 7-14 days, B. 3-5 days, C. 21-25 days, D. Less than 3 days', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]