Our maths teacher set so difficult an examination problem none of us worked out.A. whichB

问题补充：

Our maths teacher set so difficult an examination problem none of us worked out.

A. which

B. that

C. as

D. so that

答案：

A. which

B. that

(答案→)C. as

D. so that

解析：句意：我们的数学老师出了一个这么难的考试题，我们中没人能做出这个题来。这里是定语从句，so……as，as引导的是定语从句，as在从句中宾语，故选C。