The English physician Edward Jenner found that if experimental subjects were deliberately infected with cowpox, which caused only a mild illness, they are immune from smallpox.

A. which caused only a mild illness, they are immune from
B. causing only a mild illness, they become immune from
C. which causes only a mild illness, they are immune to
D. causing only a mild illness, they became immune from
E. which caused only a mild illness, they would become immune to

E