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

问题补充：

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

A. that

B. as

C. so that

D. which

答案：

(答案→)A. that

B. as

C. so that

D. which

解析：so. . . that. . . 状语从句.