B考查as引导的定语从句
as可以代替整个句子或一件事,引导非限制性定语从句,在从句中作主语、宾语或表语,意为“正如……,就像……,据……”；从句位置可置于主句之前或之后,有时也可作为插入语置于句中,一般用逗号和主句隔开.
as的这种用法常见于以下这些结构:as we can see, as
can be seen, as we know, as is（well） known, as may be imagined, as often happens, as has
been said before, as was mentioned above, as is often the case, as/indeed it
is…
例:As is well known,great changes have taken place in China. 或Great changes, as is well known, have taken place in
China.众所周知,中国发生了巨大的变化.( as在定语从句中作主语.)