正确答案应该是D（As）吧,这是一个as的固定用法.比如：As is so often pointed out ,knoledge is a two-edged weapon that could be used both for good and evil.(参见新概念第四册knoledge and progress一课)
你再看看正确答案!
你可以在百度里输入一下“as is reported(said,pointed out,told等等)”的用法,这样就清楚了!