The maths problem remained confusing to the boy although over and over.A. explainingB. be

问题补充：

The maths problem remained confusing to the boy although over and over.

A. explaining

B. being explained

C. explained

D. having explained

答案：

A. explaining

B. being explained

(答案→)C. explained

D. having explained

解析：although引导让步状语从句，主从句主语一致，从句谓语动词为be动词，可将主语和谓语动词be一并省略,补全后为:...although it was explained...。