题目:
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.
选项:
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
答案:
E