Bad habits, formed(形成), are difficult to give up.A. andB. onceC. forD. but

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A. and
(答案→)B. once
C. for
D. but
解析：考查状语从句的省略式，英语中如果主从句的主语一致，可以省略从句的主语和be动词，once formed＝once bad habits are formed句意：坏习惯，一旦形成，就很难戒掉。选B。