is known to the world,Mark Twain was a great American writer．

is known to the world,Mark Twain was a great American writer．
A、As
B、Once
C、That
D、It
【正确答案】：A
【题目解析】：【考情点拨】考查定语从句。 【应试指导】句意：众所周知，马克·吐温是一位伟大的美国作家。as在该句中引导非限定性定语从句，在从句中作主语。