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: Microbiology
Causes of antigenic drift in influenza viral infections? 
Select and explain. 
A. Small mutation in neuraminidase and hemagglutinin, B. Large mutations in hemagglutinin only, 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: Ans. is 'a' i.e., Small mutation in neuraminidase and hemagglutinin