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
Which of the following factors helped in smallpox eradication ? 
Select and explain. 
A. 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
D: Life-long immunity after disease
Answer: D: Life-long immunity after disease 
Explanation: None