____is the center of our planetary system was a difficult concept to grasp in the Middle Ages.
A Being the sun and not the earth
B That the sun and not the earth
真正的答案是选B
我想问一下：动词不定式 和 名词性从句作主语 在这道题上有什么区别呢?
为什么A 不行?
那如果说 这道题改成
____is the center of our planetary system
AB 两项 都可以么？动词ing形式做主语也可以啊！