题目内容：
根据下面内容，回答题 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 textmessages are sent each month.
The plan comes from the Warning Alert and Response Network Act, a 2006 federal law thatrequires improvements to the nation's emergency alert system. The act tasked the FederalCommunications Commission (FCC) with coming up with new ways to alert the public aboutemergencies.
"The ability to deliver accurate and timely warnings and alerts through cell phones and othermobile services is an important next step in our efforts to help ensure that the American public hasthe 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, butit has received solid support from the wireless industry.
The program would be optional for cell phone users. They also may not be charged forreceiving 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 ornatural disaster. The second would involve "approaching threats" which could include naturaldisasters like hurricanes or storms or even university shootings. The third would be reserved forchild abduction (绑架) emergencies, or so-called Amber Alerts.
The service could be in place by 2010.
What is the purpose of the approved plan? A．To warn people of emergencies via messages
B．To popularize the use of cell phones
C．To estimate the monthly number of messages
D．To promote the wireless industry
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