直流输电系统在经济上（）。
A.线路造价低
B.运行损耗小
C.特别适合电缆输电
D.远距离输电成本低于交流送电
参考答案：A, B, C, D
The elderly ______ good care of in public places like supermarkets and airports; they are likely to get lost.
A．can have been taken
B．ought to be taking
C．must be taken
D．should take
简述数控机床CNC装置硬件、软件的组成。
