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

直流牵引电动机为什么采取串励的形式？