several times，but unfortunately，the boy still couldn’t smooth away the problem on his own.
A. Having been explained
B. The problem having been explained
C. The problem had been explained
D. Although the problem had been explained
网友回答
A. Having been explained
B. The problem having been explained
(答案→)C. The problem had been explained
D. Although the problem had been explained
解析：由连词but可知，前面应该是一个完整的句子，只有C项符合。若去掉but，答案可选B：分词的独立结构，也可以选D：although引导的让步状语从句。