问题
单项选择题
Packet-switching wireless networks are preferable (71) when transmissions are (72) because of the way charges are (73) per packet. Circuit-switched networks are preferable for transferring large files or for other lengthy transmissions because customers are (74) for the (75) of time they use the network.
A．point
B．start
C．length
D．end
答案
参考答案：C
解析：
[分析]： 无线分组交换网只有(only)在短(short)时间传输时才能体现其优越性，这是因为它的计费方式是按每个分组收费(incurred)。电路交换网则适用于大型文件传输或其他长传输，因为对客户是按他们使用网络的时间长度(length)来收费(charged)的。