She is the only one of the students able to answer this question.A. who isB. who areC. th

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(答案→)A. who is
B. who are
C. that is
D. that are
解析：本题修饰一个表示人的代词one，是一个定语从句，修饰人关系词用who，who在从句中作主语，先行词为单数，故who后的be动词用is，故本题选A。