问题
单项选择题
Packet-switching wireless networks are preferable (1) when transmissions are (2) because of the way charges are (3) per packet. Circuit-switched networks are preferable for transferring large files or for other lengthy transmissions because customers are (4) for the (5) of time they use the network.
4（）
A．charged
B．fined
C．free
D．controlled
答案
参考答案：A