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

问题补充：

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

A. Which

B. It

C. That

D. As

答案：

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。