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阅读理解
What do you usually use mobile phones for? Sending text messages for fun? Playing games? Well, mobile phones can do far more than that.
During this year’s Asian tsunami (海啸), mobile phones saved many lives.
People were using more than 10,000 international phones when the tsunami hit.
Police sent text messages to them and found more than 2,000. But how? They followed the signal (信号) and found out where the tourists were.
So, how can something so small be so smart? Well, here’s how.
When you turn the phone on, it tries to find a signal. This signal comes from the telephone network. When the phone is finding a signal, it also sends a message to the telephone network. This message tells the station where the phone is.
When you start to make a phone call, the telephone network tells telecommunication (电信) offices where you are. The office will find out who you’re calling, then put you through to your friend. When you start to talk, a device (装置) in the phone changes your voice into electric signals. These signals travel across the telephone network and reach your friend’s phone. Then his phone changes the signals into your voice again. Finally, you can hear each other!
( )20. What’s the main idea of this passage? It tells us...