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;
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