a bad habit is formed it will take a long time to get rid of itA. Ever sinceB. WhileC. O

问题补充：

a bad habit is formed, it will take a long time to get rid of it．

A. Ever since

B. While

C. Once

D. Even if

答案：

A. Ever since

B. While

(答案→)C. Once

D. Even if

解析：考查连词的用法。句意：一旦坏习惯形成，它要花好长一段时间才能摆脱。Once 一旦。故选C。