城市轨道交通机电设备由消防自控系统、（）、环控系统、给排水系统、低压配电及照明系统、电梯与自动扶梯系统、屏蔽门系统及防淹门系统组成。

Unless you work as hard as you can, you won’t ______ your classmates.

A．fall behind

B．keep on with

C．keep up with

D．catch up