is mentioned above that the number of the students in senior high school is increasing. A

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A. Which
(答案→)B. It
C. That
D. As
解析：考查it做形式主语：句意：上面提到的是高中的学生数量在增加。It是形式主语，that引导的是主语从句，which和as应该引导非限制性定语从句，that引导的是主语从句，选B。