由as, which 引導的非限定性定語從句，as和which可代整個主句，相當于and this或and that。As一般放在句首，which在句中。

As we know, smoking is harmful to one’s health.

The sun heats the earth, which is very important to us.

典型例題

1)Alice received an invitation from her boss, ___came as a surprise.

A. it B. that C. whichD. he

答案C. 此為非限定性從句，不能用 that修飾，而用which.，it 和he 都使后句成為句子，兩個獨立的句子不能單以逗號連接。況且選he句意不通。

2)The weather turned out to be very good, ___ was more than we could expect.

A. whatB. whichC. thatD. it

答案B。which可代替句子，用于非限定性定語從句，而what不可。That 不能用于非限定性定語從句，it不為連詞，使由逗號連接的兩個句子并在一起在英語語法上行不通。

3)It rained hard yesterday, ____ prevented me from going to the park..

A. thatB. whichC. as D. it

答案B.

as 和which在引導非限制性定語從句時，這兩個關系代詞都指主句所表達的整個意思，且在定語從句中都可以作主語和賓語。但不同之處主要有兩點：

(1) as 引導的定語從句可置于句首，而which不可。

(2) as 代表前面的整個主句并在從句中作主語時，從句中的謂語必須是系動詞;若為行為動詞，則從句中的關系代詞只能用which.。

在本題中，prevent由于是行為動詞，所以正確選項應為B。

As 的用法

例1. the same… as;such…as 中的as 是一種固定結構, 和……一樣……。

I have got into the same trouble as he (has).

例2. as可引導非限制性從句，常帶有’正如’。

As we know, smoking is harmful to one’s health.

As is know, smoking is harmful to one’s health.

As是關系代詞。例1中的as作know的賓語;例2中，它充當從句的主語，謂語動詞know要用被動式。