No one can attain great achievements ________ he concentrates on what he is doing, even though a talent．

1. A.
   because
2. B.
   while
3. C.
   unless
4. D.
   if

No one can attain great achievements          he concentrates on what he is doing, even though a talent.                                                    

A. because           B. while             C. unless           D. if