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

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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...。