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': 'Which of the following factors helped in smallpox eradication ? \nSelect and explain. \nA. Short incubation period of disease, B. Slow evolution of rashes over body, C. Long-term carrier stages in patients, D. Life-long immunity after disease', 'role': 'user'}, {'content': 'Answer: D \nExplanation: None', 'role': 'assistant'}]