____is known to the world , Mark Twain is a gread American writera thatb it c whichd as 为什么B不对

____is known to the world , Mark Twain is a gread American writer
a that
b it
c which
d as 为什么B不对

正确答案是 D
这不是强调句型,也不是形式主语!此句的主句是Mark Twain is a gread American writer.
As is known to the world ,Mark Twain is a gread American writer 这个句子是定语从句,由as引导.