Learning another language is like stepping out of a door, ________ I can look back and see more clearly my own language．

1. A.
   by which
2. B.
   which
3. C.
   through which
4. D.
   on which

Learning another language is like stepping out of a door, ________ I can look back and see more clearly my own language.

   A. by which                                B. which

   C. through which                        D. on which