A. for multiple ISDN connections B. for modem concentrated dial-up connections C. for a group of users in the same vicinity sharing a connection D. for use by mobile user
A. 为避免让服务器增加不必要的负担 B. 避免计算机因为不断转向邮件处理程序而降低效率 C. A和B D. 接收新邮件发出的声音会干扰正常工作