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
Secular trend in Influenza due to 
Select and explain. 
A. Antigenic shift, B. Antigenic drift, C. Endemicity, D. None of the above
A: Antigenic shift
Answer: A: Antigenic shift 
Explanation: None