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
Causes of antigenic drift in influenza viral infections - 
Select and explain. 
A. Small mutation in neuraminidase and hemagglutinin, B. Large mutations in viral genome, C. Step mutations in viral genome, D. None of the above
A: Small mutation in neuraminidase and hemagglutinin
Answer: A: Small mutation in neuraminidase and hemagglutinin 
Explanation: Antigenic drift is due to mutation and selection. REF:ANANTHANARAYAN AND PANIKER'S TEXTBOOK OF MICROBIOLOGY 9TH EDITION PAGE NO-499