The pollution problem here seems to get more and more serious .Something ____with it .

A．has done

B．must be done

C．is doing

D．was done

显微镜对光时应首先（　　）

A．转动低倍物镜，使其对准通光孔

B．转动高倍物镜，使其对准通光孔

C．转动转换器，使低倍物镜对准通光孔

D．转动转换器，使高倍物镜对准通光孔