Last term our maths teacher set so difficult an examination problem none of us worked out

问题补充：

Last term our maths teacher set so difficult an examination problem none of us worked out.

A. that

B. as

C. so that

D. which

答案：

A. that

(答案→)B. as

C. so that

D. which

解析：as引导定语从句修饰examination problem,在从句中作宾语,因其先行词前有限定词so。若看作so…that…结构,结果状语从句应加宾语,为worked it out。