A.the US federal government
B.mobile phone users
C.the carriers themselves
D.the law of the United States
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.
A.a child loses his way
B.a university shooting happens
C.a natural disaster happens
D.a terrorist attack occurs
A.It must appear in time.
B.It must be done in class.
C.It must be done on a construction sheet.
D.It must include the words on the blackboard.