问题详情

Packet-switching wireless networks are preferable(66)when transmissions are(67)bemuse of the way charges are(68)per packet. Circuit-switched networks are preferable for transferring large files or for other lengthy transmissions because customers are(69)for the(70)of time they use the network.
A．to
B．for
C．than
D．only请帮忙给出正确答案和分析，谢谢！