as引导非限定性定语从句

Question

___ is mentioned above, the number of the students of senior high schools is increasing.
A. Which
B. As
C. That
D. It
选B，为什么
老师说，只要看到句首有空就只能填as，为什么，具体讲讲

Answer 1

which与as引导非限制性定语从句的异同：
一、相同点
两者引导非限制性定语从句时，可指代主句中的部分或整个句子的内容，在从句中做主语，宾语或表语，位于主句之后有时可互换。
The meeting was put off, as /which was exactly what we wanted.
He was a doctor, as/which I knew from his manner.
二、不同点
1. as可以放在主句前后，也可插入主句之中；而which引导的非限制性从句只能位于主句之后。
As is known to all, fish can’t live without water.
Air, as we know, is gas.
2.在非限制性从句中，which后的be动词不可省；as而后的be动词可以省略。
She told me she won the match, which was a lie.
The material is elastic, as (was) shown in the figure.
3. which在非限制性定语从句做主语时可用各类动词做谓语；而as做主语时，谓语常用系动词，如be, seem, become等，一般不用其他行为动词。
He saw the girl, which delighted him.
He didn’t say anything at the meeting , as/which seemed very strange.
4. as常用“正如”含义，常用的结构有as we know（众所周知）；as often happens（正如常发生的那样）；as is often the case（情况常常如此）; as we all can see（正如我们看到的）; be announced/expected/known/imagined/mentioned/said/shown/
reported等。这些结构常放句首，偶尔也可以放句中或句末。
As is known to all, China is a developing country.
Kate was late for school, as often happened.
5. 在非限制性定语从句中which指代主句中某一个单词时，as不可以。
My brother enjoyed playing basketball, which he really plays well.
Beijing, which he was born in, is our capital.
6. “介词+关系代词（介宾代物）”中关系代词只能用which。
The Travel Agency, with which our company has been dealing for several years, has opened for new branches.
Air is a mixture of gases, of which oxygen forms 21 percent.
7. 从句中的谓语动词是否定形式时，常用which。
He can write a letter in English, which I can not.
Metal will bear beating with a hammer, which a stone will not
8.在非限制性定语从句关系代词代表前面主句中的宾语从句时，常用which
He said he had passed the exam, which was untrue.

Answer 2

从以下几点来剖析此题目：
1原题中的几个非限制性定语从句的特征：
a）两句之间有逗号隔开b）去掉逗号前的句子，后面的句子意思完整无误才c）从该从句所修饰的部分来看，不是一般定语从句中的某个先行词，而是整个句子或句子的一部分
2由以上断定为非限制性定语从句
3非限制性定语从句的关系代词有：
a）指代人的有：who、whom、of whom、whose等；
b）指代物的有：which、as、of which、whose等；
4结合本题，很明显从句的位置在句首，这个时候得考虑which和as的使用区别：【as引导非限制性定语从句，若as在从句中作主语，其引导的句子可以放在句首，也可以放在句中。但which引导的非限制性定语从句只能放句中。 】
5从以上看来，答案为as！

希望以上分析，你可以完全掌握这类题目！