As is known to all ,和Which is known to all,China is a social

as is known to all通常是句首语,起到引导下面内容的作用；which is known to all通常是一个定语从句,跟在某一句子或者某个词后面,起到解释说明的作用.题中As is known to all,China .意思是：众所周知的是,中国是一个社会主义国家.而Which is known to all,China is .意思是：就如大家都知道的,中国是一个社会主义国家.