David can speak idiomatic American English and yet English he doesnt know it at all. A

问题补充：

David can speak idiomatic American English and yet, English, he doesn know it at all.

A. how to teach

B. when it comes to teaching

C. when he teaches

D. however he teaches

答案：

A. how to teach

(答案→)B. when it comes to teaching

C. when he teaches

D. however he teaches

解析：when it comes to doing sth.是惯用法，意思是“当提到……的时候，”符合题意，而其它选项在逻辑上与上下文不符。