一个英语语法选择题_____is the center of our planetary system was a di

一个英语语法选择题
_____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
为什么?D
为什么不可以呢
不太明白啊，如果it是做形式主语的话，那么it is the sun and not the earth这句句子 和后面地is the center of our planetary system 是怎么回事啊？如果它是后面一句的主语吗？(=@__@=) 以前好像老师说过，只有从句如 what 之类的引领的句子才能作为其他人句子的主语。
如果it is 是强调的话，那不就出现多个谓语了吗？