Smoking, is a bad habit, is popular with the young.A. thatB. whichC. whatD. as

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A. that
(答案→)B. which
C. what
D. as
解析：考查定语从句。本题是非限制性定语从句，which指代smoking在句中做主语。That不能引导非限制性定语从句。What不是定语从句的关系词，as有正如的意思，与本句句意不符。句意：抽烟是一个不好的习惯，很受年轻人欢迎。故B正确。