Question

_________to all is that Taiwan is the largest island in China.

A.As is known B.As we know C.It is known D.What is known

Answer 1

选D
整个句子的结构是表语从句，is 做系动词，that 引导表语从句。所以根据主系表结构，is前面应该缺的是句子的主语。选项中只有D可以做主语，因为D是主语从句。
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Answer 2

D

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Answer 3

C.It is known

Answer 4

选D。根据all后面的is判断前面是what引导的主语从句。