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

问题补充：

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

A. It

B. That

C. What

D. As

答案：

A. It

B. That

C. What

(答案→)D. As

解析：考察定语从句。关系代词As指代后面一句话的内容在句中作主语。句意：正如上面所提到的那样，高中学生的数量一直在增加。