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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。
1、谓语动词是found，不表示宾语从句一定用过去式，宾语从句发生在过去，用过去式；如果是惯常的事情用一般现在时，如：Scientists discovered that the earth revolves around the sun.
2、除虚拟语气外的if/then的时态：
if用一般现在时，表示在事情发生的情况下，then用将来时，”主将从现“。如：
If it doesn’t rain tomorrow, we’ll go to the Great Wall.
没有if过去式，过去的事情是不可改变的，如果对过去的事情进行if，表示假设与过去的事情不同另一种情况，即与过去的事情相反的情况，if过去的事情自动转入虚拟语气。
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