as与which均可替代整个主句
在非限制性定语从句中，均可替代整个主句. 如从句在主句之后，两者皆可用；如从句在主句之前，用as。 介词/逗号后, 永远不用that!
they failed in the exam，as/which is natural.
as is known to all，the earth moves around the sun.
he passed the college entrance examination, made his parents very happy.
a. as b. which c. that d. it
