关于公用电话网中使用的信号音我国多采用（）Hz的频率，通过不同的断续方式来定义。
A、450
B、550
C、650
D、750
参考答案：A
-----Are you still very busy?
-----Yes, I ______ the report for the manager and it won’t take long.
A．have just finished
B．am just finishing
C．had just finished
D．am just going to finish
饭店电气设备防火应注意哪些？
