9. It is such a difficult problem _______ no one can work out.

问题描述:

9. It is such a difficult problem _______ no one can work out.
A. asB. thatC. soD. thAT
答案怎么是A,应该B把

如果选B的话
从句应该是no one can work it out.
本句是such...as句型