Question

____is known to the world ,Beijing successful held the 29th Olympic Games. A.It B.As 为什么选B

Answer 1

1、As is known to all 是由as引导的定语从句的形式 It is known to all 是由It做形式主语而引导的句子
As is known to the world ,Beijing successful held the 29th Olympic 此时AS是作为非限制性定语从句的引导词
It is known to the world that Beijing successful held the 29th Olympic . 此时it只是个形式主语，真正的主语是由后面that引导的句子。

Answer 2

这是As的固定用法，是由于的意思