This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Virology, Subject name: Microbiology
[{'content': 'Causes of antigenic drift in influenza viral infections - \nSelect and explain. \nA. Small mutation in neuraminidase and hemagglutinin, B. Large mutations in viral genome, C. Step mutations in viral genome, D. None of the above', 'role': 'user'}, {'content': "Answer: A \nExplanation: Antigenic drift is due to mutation and selection. REF:ANANTHANARAYAN AND PANIKER'S TEXTBOOK OF MICROBIOLOGY 9TH EDITION PAGE NO-499", 'role': 'assistant'}]