有读者问： 有这样一道选择题：
(1)_____has been announced, we must hand in our term paper before the tenth of June.
A. It B. That C. As D. What
答案为C，但A为什么不可以选？
不可选A，因为it不知何所指。选C是因为as是关系代词（其所引导的从句可以前置），其先行词是主句we must hand in our term paper before the tenth of June。关系代词as在其所引导的这种非限制性定语从句中用作主语，其谓语动词一般是be或连系动词，再如：
(2)As is announced in today's papers, all the schools will reopen on 15 February.
(3)As is often the case, she was the first to come to class.
(4)He married again, as seemed natural.
关系代词as指代主句时，还可在其所引导的这种非限制性定语从句中用作宾语，如：
(5)As you will find out, the whole village is rebuilt.
