as和 which都可指代前面一句话，请看例句:
1）Alice received an invitation from her boss, which came as a surprise.
爱丽丝受到了她老板的邀请，这使她感到吃惊。
2）The weather turned out to be very good, which was more than we could expect.
天气结果变好了，这是我们没有预料到的。
若有否定as错；
3）The result of the experiment was very good, _______ we hadn't expected.
A. when B. that C. which D. what
【简析】答案是C。由于非限定性定语从句是否定结构，只能用which引导，不能用as来替代which。
句首只能用as,还有认知猜想词；
置于句首时，非限定性定语从句只能用as引导。例如：
As is known to everybody, the moon travels round the earth once every month.
众所周知，月亮每月绕地球一周。
与表示认知猜想的词，如know, expect, see, report, 连用时，要用as。
1）As we know, smoking is harmful to one's health.
我们知道，吸烟对健康有害。
2）As is expected, his daughter entered a key university.
正如人们所期望的那样，他的女儿考入了一所重点大学。
固定结构用as, the same /such/so/as;
1） 在the same …as….结构中。意思是 “像……一样的”。 例如：
Chongqing is not the same city as we have seen in other places.
重庆和我们在其它地方看过的城市不一样。
注意区分: the same ….that….,请看例句：
This is the same pen that I lost. 这正是我所丢的钢笔。（同一）
This is the same pen as I lost. 这支钢笔和我丢的那支一样。（相似，不是一个）
2）在as...as结构中，意思是“像……那样的”。例如：
No period in history has had as many important changes _______ have taken place in the past century. 没有哪个历史阶段像过去的一个世纪这样有如此多的重大变化。
3）在such…as…结构中, 意思是“像……那样的”。例如：
Mathilde had to work hard day and night and bought such a diamond necklace as she had lost to pay back to Jeanne.
马帝尔德不得不日日夜夜地干活，来买一个像被她丢掉的那样的钻石项链还给让。
So /such …that宾不离，so/such…as宾要弃；
在so/such…that结构中，that后引导的不是定语从句，而是状语从句，从句中必须有宾语；在so/such…as结构中， as后引导的是定语从句，从句中不带宾语。
例如：
1)He studies in such a good school that all other students in his village admire him.
他在那样好的一所学校学习，以至于村子里其他的学生都很羡慕他。
解析：that后是状语从句，状语从句中有宾语him。
2）He studies in such a good school as all other students in his village admire.
他在那样好的一所学校学习，村里的其它学生都很羡慕那所学校。
解析：as后是定语从句,as代替先行词school,在定语从句中做admire的宾语。
