As a teacher,  I seldom give my students so difficult a problem ________ they can not work out.

As a teacher, I seldom give my students so difficult a problem ________ they can not work out．

1. A.
   that
2. B.
   if
3. C.
   in order that
4. D.
   as

As a teacher,  I seldom give my students so difficult a problem ________ they can not work out.

A. that　　　　 B. if          C. in order that　　　　　 　D. as

As a teacher, I seldom give my students so difficult a problem ________ they can not work out.

A. that　　　　B. if     C. in order that　　　D. as