Helping others is a habit,you can learn even at an early ageA. itB. which C. what D. that

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A. it
(答案→)B. which
C. what
D. that
解析：考查定语从句：句意：帮助别人是个习惯，你可以从早年就学习。这是非限制性定语从句，先行词是a habit，定语从句中缺少宾语，应该是关系代词，that不能引导非限制性定语从句，选B。