David can speak idiomatic American English and yet, _____ English, he doesn't know it at all.

1. A.
   how to teach
2. B.
   when it comes to teaching
3. C.
   when he teaches
4. D.
   however he teaches

David can speak idiomatic American English and yet, _____ English, he doesn't know it at all.

       A. how to teach               B. when it comes to teaching

C. when he teaches                     D. however he teaches