同意曹老师的解答，补充几点以供参考：

1.当定语从句位于句尾表“正如......; 就像......”时要用as，而which则意为“这件事；这一点” (相当于and this / that / it)，它们所表达的意义是有差异的。试比较：

Everyone likes Tom, as you can see. 正如你所看到的，大家都喜欢汤姆。

Everyone likes Tom, which you can see. 大家都喜欢汤姆，这一点你是能够看得出来的。

2.当定语从句位于句尾，谓语动词是be或其它的连系动词 (如seem, become等) 时，可用as或which作主语；但当从句的谓语是行为动词时，只能用which作主语。例如：

He failed in the driving test, which / as was natural. 他驾考挂科了，这是很自然的事。

Moyan won the Nobel Prize in literature, which made him illustrious. 莫言获得了诺贝尔文学奖，这使得他名扬天下。

3.当定语从句位于句中指代某个名词或代词而不指代整个句子时，不能用as。例如：

The flat, which I bought last year, has not been decorated. 这个公寓是我去年买的，还没装修呢。