定语从句之专题三：as引导定语从句（系统讲解内在原理）

题组29（as/it/what）

1. _____ is known to everybody, the moon travels round the earth once every month.（全国卷）

A. It   B. As   C. That   D. What   选B。

2. The air quality in the city, ____is shown in the report, has improved over the past two months. (2012,福建)

A. that   B. it   C. as   D. what

句意：正如报告中所写的那样，在过去的两个月中，这座城市的空气质量提高了。

分析：把从句放在句首就很明白了:    is shown in the report, the air quality in the city has improved over the past two months. “正如在报告中所写的那样”，as指的是下文所提到的话。选C。

3. ____ in the regulations that you should not tell other people the password of your email account.（上海高考题）   A. What is required   B. What requires is   C. It is required   D. It requires  

句意：规则要求你不应把你的电子邮件的密码告诉别人。选C。

4. ____ was most important to her, she told me, was her family. (2008,山东).

A. It   B. This   C. What   D. As   

分析：what是主语从句的引导词，其余都不是。选C。

5. Everyone knows that ____ is dangerous to play with fire，but ____ is difficult is to prevent children from playing with fire. (陷阱题).   A. it, it   B. what, what  C. it, what  D. what, it

分析：容易误选A或B。其实，第一空填形式主语it，真正的主语是其后的不定式 to play with fire。第二空填 what，what is difficult 是主语从句（注意到what is difficult 后有is，若没有这个is，则第二空也填it）。答案是C。