several times，but he still doesn't know how to do it correctly.
A. Although I've taught him
B. I've taught him
C. Having been taught
D. Having taught
网友回答
A. Although I've taught him
(答案→)B. I've taught him
C. Having been taught
D. Having taught
解析：although不能与but连用，故不能选择A。若选C，必须把并列连词but去掉，这样就变成了一个简单句，为现在分词短语作让步状语。