____ caused the accident is still a complete mystery. A. What B. That C. How D. Where
答案为 A。但是,我不明白为什么不能选 B(That)。难道 that 不能引导主语从句吗?
虽然 what 和 that 都可以引导名词从句(主语从句、表语从句和宾语从句)，但它们有一个本质上的差别，就是 that 是个连词，不能在名词从句中充当句子成分，而 what 除引导从句之外，还要在它所引导的从句中充当句子成分(主语、表语或宾语)。在上面的试题里之所以必须选 What，就是因为除了用它引导一个主语从句作句子谓语 is 的主语之外，还要用它在从句中作从句的主语(其谓语是 caused)，其意义相当于 the thing(s) that 或 that which。请对比 what 和 that 引导的主语从句：
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What that child needs is a good spanking! (what 引导主语从句并在从句中作 needs 的宾语) 那孩子需要的是被狠狠地打一顿屁股!
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What we like to do most on holiday is just to visit old churches. (what 引导主语从句并在从句中作 to do 的宾语) 我们在假期最喜欢做的事情就是参观古老的教堂。
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What he says is true. (what 引导主语从句并在从句中作 says 的宾语) 他所说的是真实的。
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What I do with this money is not your concern. (Dan Brown, The Da Vinci Code) 我怎样处理这笔款子与你们无关。(《达·芬奇密码》)
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It was what they had been working so hard to find: pure radium. (what 引导表语从句，并在从句中充当 find 的宾语) 这就是他们一直辛勤劳动、孜孜以求的东西: 纯镭。
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That he will help us is certain. (that 引导主语从句,但在从句中不作任何句子成分,仅仅是个连词) 他将会帮助我们,那是肯定无疑的。
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That John became a poet may have been due to his mother's influence. 约翰成为诗人，可能是受了他母亲的影响。
请做下面的英语试题：
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In some countries, ____ is called “equality” does not really mean equal rights for all people. A. which B. that C. what D. one
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What a different man he is ____ he was 4 years ago! A. from what B. with that C. to whom D. by whom
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Perseverance is a kind of quality — and that's ____ it takes to do anything well. A. what B. that C. which D. why(上海高考,02-40)
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——Are you still thinking about yesterday's game? ——Oh, that's ____. A. what makes me feel excited B. whatever I feel excited about C. how I feel about it D. when I feel excited(北京高考,03春-25)
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____ made the school proud was ____ more than 90% of the students had been admitted to key universities. A. What; because B. What; that C. That; what D. That; because(上海高考,03春-33)
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Although Anne is happy with her success she wonders ____ will happen to her private life. A. that B. what C. it D. this(全国四级,00.1-50)
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This book will show the readers ____ can be used in other contexts. A. how that they have observed B. that how they have observed C. how what they have observed D. that they have observed(全国六级,96.1-49)