The Technique of Vaccination as Developed by Jenner Was an Improvement

The technique of vaccination as developed by Jenner was an improvement over variolation because

A) the smallpox virus mutates frequently but the cowpox virus does not.
B) cowpox vaccination was not just preventative but could also cure an active case of smallpox.
C) cowpox was common in Europe at the time while smallpox was not.
D) the cowpox virus used by Jenner was not fatal to humans but provided protection against smallpox.
E) Jenner's technique required a lower rate of herd immunity to prevent the spread of smallpox.