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

（）是应收电费汇总凭证的原始依据，既是电力企业经营成果的反映，也是抄表人员当日工作的记录。

A.收费报表

B.抄表日报

C.总分类账

D.明细账