这个是说which和as引导非限制性定语从句的区别:
1.which是只能放在句子中,即前面必须有一个句子:He tore up my photo, which upset me.
2.as是既可以放在句首,也可以放在句中:
As has been mentioned above, computers have many advantages.
Einstein, as we know, is a famous scientist.
There was a net bar around here, as I remember.
所以基本上就As放在句首吧.