______ the sun and not the earth is the center of our planetary system was a difficult concept to grasp in the Middle Age.
A、
Why
B、
What
C、
Where
D、
That
【正确答案】
D
【答案解析】
that引导主语从句，在句中不充当任何成分，只起引导词的作用。
提交答案
关闭