as ／which引導非限制性定語從句.
引導詞as和which的區(qū)別在于：
①as引導的非限制性定語從句可放于主句前、主句中、主句后,而which引導的非限制性定語從句一般放于主句后或句中.as放在句首的情況常用的句式 有as we all know,as is mentioned above,而which我們則看不到這樣用于句首的情況.
②as常與從句中的know,see,hear,expect等動詞連用,也常用于as often happens,as is often the case（常有的事）等句子中；而which一般不用于以上情況.
③as有“正如”的含義,which沒有此含義.如：
As is well known,Shanghai is a big city．
Some of the roads were flooded,which made our journey more difficult．