【正确答案】 D

【答案解析】[解析] 本句为as引导的非限制性定语从句。也可将as is known to all(众所周知)当作固定搭配。类似的用法还有as is often the case(情况往往是这样)，as often happens(事情往往如此)，as is mentioned above(正如上面所提到的)等。故选D。