基本信息
2001年高考中有这么一道单项选择题：
is known to everybody, the moon travels round the earth once every month.
A. It B. As C. That D. What
此题的答案为B, as 在这引导一个非限制性定语从句，修饰后面这句内容。但有学生不明白为什么不选A或D。这就说明他们对一些句型结构分辨不清，我们把题目做以下变化：
(1) is known to everybody that the moon travels round the earth once every month.
(2) is known to everybody is that the moon travels round the earth once every month.
