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

答案及解析： D

as引导定语从句。关系代词as在从句中作主语或宾语。如果选A项that引导的是状语从句，状语从句中that不作成分所以句子应是“that they can not work it out”