问题
Which of the following is TRUE of cell phone users？
A:They must accept the alert service．
B:They may enjoy the alert service for free．
C:They must send the alerts to others．
D:They may choose the types of messages．
B:They may enjoy the alert service for free．
C:They must send the alerts to others．
D:They may choose the types of messages．
正确答案是：
