电源系统中通常使用什么颜色来分别标识三相电源？（）
A.红、黄、绿
B.红、黄、蓝
C.黄、绿、蓝
D.红、黄、黑
参考答案：B
It’s reported that the police will soon ____ the case of two missing children.
A．look into
B．look after
C．look upon
D．look out
