______ is the center of our planetary system was a difficult concept to grasp in the Middle Age.

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问题 ______ is the center of our planetary system was a difficult concept to grasp in the Middle Age.

选项 A、It is the sun and not the earth
B、Being the sun and not the earth
C、The sun and not the earth
D、That the sun and not the earth

答案D

解析 题型:语法类试题。根据句子结构我们可以看出,在was前面是一个主语从句,作为整个句子的主语,因此该从句中缺少一个引导词。而常作引导词的有what,that,which等,A,B,和C中都没有引导词,因此三个均错。 D中that引导主语从句,在句中不做任何成分,只起引导词的作用,因而D是正确答案。
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