What is the main idea of the news item？A．The United States developed a computer chip for c
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What is the main idea of the news item？
A．The United States developed a computer chip for cell phones.
B．A new cell phone emergency alert system will be launched in the U.S.
C．Local cell phones towers have been built in many cities of U.S.
D．Dangerous messages could be conveyed through cell phones.
请帮忙给出正确答案和分析，谢谢！
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正确答案：B
本题问及新闻的主要内容是什么。新闻指出，美国引入了一种新的公共警报系统，该系统能在紧急状态将警报发送至移动电话。并允许手机免费获取迫在眉睫的安全威胁警报。故正确答案是B。新闻的首句一般为主旨句。