2. as单独引导定语从句

as单独引导定语从句时，先行词可以是一个词，也可以是一个句子或短语。例如：

(1) She is late, as is often the case.她迟到了，这是经常的事。(先行词是整个主句)

(2) To shut your eyes to facts, as many of you do, is foolish.对事实视而不见--你们好多人都是如此的愚蠢。(先行词是不定式短语)