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
网友回答
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。