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
The protection against small pox by previous infection with cowpox represents - 
Select and explain. 
A. Antigenic cross-reactivity, B. Antigenic specificity, C. Passive immunity, D. Innate immunity
A: Antigenic cross-reactivity
Answer: A: Antigenic cross-reactivity 
Explanation: None