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': 'Major epidemics are due to antigenic drift \nSelect and explain. \nA. Asymptomatic seen rarely, B. Incubation period 10-12 hrs, C. All ages and sex equally affected, D. Pandemic rare', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]