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

【正确答案】 D

【答案解析】[解析] 根据句子结构我们可以看出，在was前面是一个主语从句，而主语从句必须有引导词，但A项、B项和C项中都没有引导词，因此三项均错。D项中That引导主语从句，在句中不作任何成分，只起引导词的作用，因而D项是正确答案，例如：That he will refuse the offer is unlikely. 他拒绝这项建议的可能性不大。