Bad habits formed are difficult and sometimes impossible to shake off.A. before B. onc

问题补充：

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

A. before

B. once

C. until

D. unless

答案：

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。