97-01,第10题,不懂

10. ____, dolphins have no sense of smell.

(A) As known as far

(B) Known thus far as

(C) It is known as far

(D) As far as is known

这个D选项,如果是As far as it is known,我觉得就对了,怎么能这么用,谁能解释一下

主语和后面一致，可以省略