__isknowntotheworldMarkTwainwasagreatAmericanwr

__ 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
【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