In the final exam of last term, our maths teacher Mr.Wu gave us we all couldn’t work out .
A. such a difficult problem that
B. so a difficult problem as
C. so difficult a problem that
D. as difficult a problem as
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A. such a difficult problem that
B. so a difficult problem as
C. so difficult a problem that
(答案→)D. as difficult a problem as
解析：本题考查状语从句。本题考查so/as + adj. + a/an + n. + as或such + a/an + adj + n. + as结构，根据其结构，故选D。