It is generally considered that the key ______a foreign language well is nothing      but practice.

It is generally considered that the key ______a foreign language well is nothing      but practice.

1. A.
   to learning
2. B.
   to learn
3. C.
   of learning
4. D.
   for learning 

It is generally considered that the key ______a foreign language well is nothing      but practice.

A. to learning B. to learn 

C. of learning D. for learning

It is generally considered that the key ______a foreign language well is nothing      but practice.

It is generally considered that the key ______a foreign language well is nothing      but practice.