问题
选择题
The maths problem remained confusing to the boy although _______ over and over.
A．explaining
B．being explained
C．explained
D．having explained
答案
C
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...。