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
[{'content': "Small pox eradication was successful because of all except- \nSelect and explain. \nA. Subclinical cases don't transmit disease, B. Highly effective vaccine -, C. Lifelong immunity, D. Cross resistance with animal pox", 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]