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as, which 非限定性定语从句
由as, which 引导的非限定性定语从句，as和which可代整个主句，相当于and this或and that.As一般放在句首，which在句中。
As we know, smoking is harmful to ones health.
The sun heats the earth, which is very important to us.
典型例题
1）Alice received an invitation from her boss, ___came as a surprise.
A. it B. that C. which D. he
答案C. 此为非限定性从句，不能用 that修饰，而用which.，it 和he 都使后句成为句子，两个独立的句子不能单以逗号连接。况且选he句意不通。
2）The weather turned out to be very good, ___ was more than we could expect.
A. what B. which C. that D. it
答案B.which可代替句子，用于非限定性定语从句，而what不可。That 不能用于非限定性定语从句，it不为连词，使由逗号连接的两个句子并在一起在英语语法上行不通。
3）It rained hard yesterday, ____ prevented me from going to the park
A. that B. which C. as D. it
答案B.
as 和which在引导非限制性定语从句时，这两个关系代词都指主句所表达的整个意思，且在定语从句中都可以作主语和宾语。但不同之处主要有两点：
