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题库 / OG2018-SC-735
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.
which caused only a mild illness, they are immune from
causing only a mild illness, they become immune from
which causes only a mild illness, they are immune to
causing only a mild illness, they became immune from
which caused only a mild illness, they would become immune to
题目分析：
略。
选项分析：
A选项：由于they are immune from smallpox是一个虚拟的情况，即，它是存在于“实验者感染cowpox”的情况下的，因此，其应该用虚拟语气，即，they would become immune from smallpox。
B选项：分词短语causing only a mild illness是伴随状语，其主语为它所修饰的句子的主语，即，experimental subjects。但在逻辑上，引起疾病的应该是 cowpox，而不是experimental subjects。
C选项：本选项错误同选项(A)。
D选项：本选项错误同选项(B)。
E选项：Correct. 本选项在语法和逻辑上均是正确的。
if也可以引导非虚拟，如果if从句加的是动词原型，表明作者觉得这个if是很可能发生的，所以后面也就不需要用虚拟语气了，只是假设一种可能出现的现象
eg： A wildlife expert predicts that the reintroduction will fail if the density of the timber wolf population in that region is greater than one wolf for every 39 square miles.
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