问题
选择题
________ 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
答案
答案：B
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。