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
[{'content': 'The protection against small pox by previous infection with cowpox represents - \nSelect and explain. \nA. Antigenic cross-reactivity, B. Antigenic specificity, C. Passive immunity, D. Innate immunity', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]