1.() is known to us all是句子主语,称作主语从句.能引导主语从句,又在从句里做主语指事或物,用what.
2.(1)如果选A项it,则去掉第二个is才可以.这样变成了it作形式主语,that the 2008 Olympic Games will take place in Beijing是真正主语.It is known to us all that the 2008 Olympic Games will take place in Beijing.
(2)如果选C项as,则变成由as引导的定语从句,但要用逗号隔开.
即：As is known to us all ,the 2008 Olympic Games will take place in Beijing.
(3)如果选D which,则也变成定语从句.which引导的定语从句不能位于句首