(1)_is known to the world,Mark Twain is an great American writer.

问题描述:

(1)_is known to the world,Mark Twain is an great American writer.
(2)_is known to the world that Mark Twain is an great American writer.
(3)_is known to the world is that Mark Twain is an great American writer.
A What B It C Which D As

as
it
what