Packet-switching wireless networks are preferable（66)when transmissions are（67)because of

Packet-switching wireless networks are preferable(66)when transmissions are(67)because 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