题目内容：
根据下列题干，回答题：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 net- work.
请选择（ 1 ）处最佳答案
参考答案：
答案解析：