Smallpox is a virus that causes a rash that then turns into painful blisters. The last known case of smallpox was in 1977. What is the best explanation for the eradication of smallpox?
A. People are exercising more than they used to.
B. Genetic testing determines who is more prone to smallpox.
C. There was an increase in antibacterial products.
D. A vaccine was developed.