Bad habits, _______ formed, are difficult, and sometimes impossible to shake off.

1. A.
   before
2. B.
   once
3. C.
   until
4. D.
   unless

Bad habits, _______ formed, are difficult, and sometimes impossible to shake off.

A. before               B. once                 C. until                 D. unless

Bad habits, _______ formed, are difficult, and sometimes impossible to shake off.

Bad habits, _______ formed, are difficult, and sometimes impossible to shake off.

A．before            B．once             C．until              D．unless

       Bad habits, ____formed, are difficult, and sometimes impossible to shake off.

A. once                 B. before               C. until                 D. unless