Jenner's successful use of cowpox virus as a vaccine against the smallpox virus is due to the fact that

A)the immune system responds nonspecifically to antigens.
B)the cowpox virus made antibodies in response to the presence of smallpox.
C)cowpox and smallpox are antibodies with similar immunizing properties.
D)there are some antigenic determinants common to both pox viruses.
E)cowpox and smallpox are caused by the same virus.