闭路监控系统由四部分构成（）
A、前端设备
B、传输部分
C、控制设备
D、显示设备
参考答案：A, B, C, D
I just wonder ____ that makes him so excited.
A．why it does
B．what he does
C．how it is
D．what it is
精馏是化工生产中常用液—液分离手段。
