As far as is known
当主语是所谓的" it," 即it并不指示具体的东西时,我们通常把as用作关系代词,as作了关系代词就相当于包含了代词it
并列句 同一个主语,不可以省略吗.but后并 不少主语我们老师说as far as it is known 可以这么用啊