Bad habits, formed, are difficult, and sometimes impossible to shake off.
A. before
B. once
C. until
D. unless
网友回答
A. before
(答案→)B. once
C. until
D. unless
解析：句意：坏习惯一旦形成，很难有时是不可能去掉的了。Before……以前；once一旦；until直到……才；unless除非。这里once后跟从句，从句是省略句，完整的是once they are formed，根据句意故选B。