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

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

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 名词性从句。整个句子缺少主语,故判断空格处为主语从句的一部分;其次,该主语从句缺少主语,再结合选项可知,此时的连接词应该是that。
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