第8题: [单项选择]______ is the center of our planetary system was a difficult concept to grasp in the Middle Age. 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 参考答案:D 答案解析:[精解] 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