---Why do you think he can succeed?---He believes in himself, is the most important, I th

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(答案→)A. which?
B. what
C. that
D. who
解析：此题考查的是非限制性定语从句，that 不能引导非限制性定语从句，what 不能引导定语从句，who 作主语指代前面的先行词“人”，which引导非限制性定语从句指前面的先行词也可指前半句话的含义，此处选which指的是前半句话的含义。