题目内容：
请根据短文内容，回答题。Cell Phones: Hang Up or Keep Talking?
Millions of people are using cell phones today. In many places it is actually considered unusual not to use one. In many countries, cell phones are very popular with young people. They find that the phones are more than a means of communication--having a mobile phone shows that they are cool and connected.
The explosions around the world in mobile phone use make some health professionals worded.
Some doctors are concerned that in the future many people may suffer health problems from the use of mobile phones. In England, there has been a serious debate about this issue. Mobile phone companies are worded about the negative publicity of such ideas. They say that there is no proof that mobile phones are bad for your health.
On the other hand, why do some medical studies show changes in the brain cells of some who use mobile phones? Signs of change in the tissues of the brain and head can be deteeted with modern scanning (扫描) equipment. In one case, a traveling salesman had to retire at a young age because of serious memory loss. He couldn't remember even simple tasks. He would often forget the name of his own son. This man used to talk on his mobile phone for about six hours a day, every day of his working week, for a couple of years. His family doctor blamed his mobile phone use, but his employer's doctor didn't agree.
What is it that makes mobile phones potentially harmful? The answer is radiation. High-tech machines can detect very small amounts of radiation from mobile phones. Mobile phone companies agree that there is some radiation, but they say the amount is too small to worry about.
As the discussion about their safety continues, it appears that it's best to use mobile phones less often. Use your regular phone if you want to talk for a long time. Use your mobile phone only when you really need it. Mobile phones can be very useful and convenient, especially in emergencies. In the future, mobile phones may have a warning label that says they are bad for your health. So for now, it's wise not to use your mobile phone too often.
People buy cell phones for the following reasons EXCEPT that__________. A．they're popular.
B．they're cheap.
C．they're useful.
D．they're convenient.
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