What is the main idea of the news item？A．The United States developed a computer chip for c

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！