“ Although ,” the boy said , “ over and over , the maths problem remained confusing to me

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(答案→)A. explained
B. explain
C. explaining
D. having explained
解析：这题考查非谓语动词和省略式，当主从句的主语一致时，可以省略从句的主语和be动词，the boy said ,是插入语，暂且不看Although explained over and over= Although ,the maths problem was explained over and over，句意是：男孩说：虽然这个数学问题被解释了一遍又一遍，它对我来说还是不理解。选A。