E Federal regulators Wednesday approved a plan to create a nationwide emergence alert (警报) system using text messages delivered to cell phones. Text messages have exploded in popularity in recent years, particularly among young people. The wireless industry’s trade association, CTIA, estimates(估计) more than 48 billion text messages are sent each month. The plan comes from the Warning Alert and Response Network Act, a 2006 federal law that requires improvements to the nation’s emergency alert system. The act tasked the Federal Communications Commission (FCC) with coming up with new ways to alert the public about emergencies. "The ability to deliver accurate and timely warnings and alerts through cell phones and other mobile services is an important next step in our efforts to help ensure that the American public has the information they need to take action to protect themselves and their families before, and during, disasters and other emergencies," FCC Chairman Kevin Martin said following approval of the plan. Participation in the alert system by carriers telecommunications companies is voluntary, but it has received solid support from the wireless industry. The program would be optional for cell phone users. They also may not be charged for receiving alerts. There would be three different types of messages, according to the rules. The first would be a national alert from the president, likely involving a terrorist attack or natural disaster. The second would involve "approaching threats," which could include natural disasters like hurricanes or storms or even university shootings. The third would be reserved for child abduction (绑架) emergencies, or so-called Amber Alerts. The service could be in place by 2010. The carriers participation in the system is determined by ______.

A. the US federal government
B. mobile phone users
C. the carriers themselves
D. the law of the United States