I always regret notharder when I was young .A. studyingB. having studiedC. to studyD. to h

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A. studying
(答案→)B. having studied
C. to study
D. to have studied
解析：考查非谓语动词。句意“我这是后悔年轻的时候没有好好学习。”regret doing sth.“后悔做了某事”；regret to do sth.“后悔要做某事”。根据句意是已经发生的事情，排除C、D；根据when从句可知，是后悔过去的事情，已经用完成时。故选B。