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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。
3、本题的were/would确实是虚拟语气。表示的是与通常情况相反的情况，不表示与科学家found的那个时点时相反的情况。举另一个例子来理解：
Scientists found that if flowers were watered with sulfuric acid, they would die.
我们可以想见，科学家一定是做了这个实验，给某朵花浇了硫酸，才能发现交了硫酸后花会死。从这个角度说，if/then的事情与科学家found这个时点发生的事情是一致的，不需要用虚拟语气。
但这个格式用虚拟语气，是要表示“花被浇硫酸”这件事情与通常情况是不符的，世界上大部分的花都好好的，不会被人浇硫酸。这也是为什么这里的虚拟用的是if did，then would，表示与现在的/惯常的情况相反。
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