边际收益等于_______。

蛛网膜下腔出血最常见的原因是（）。

A.外伤
B.高血压动脉硬化
C.血液病
D.先天性脑动脉瘤
E.脑血管畸形

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Cell Phones:Hang Up or Keep Talking? Millions of people areusing cell phones today. In many places it is actually considered unusualnot to use one. In many countries, cellphones are very popular with young people. They find that the phones are morethan a means of communication-having a mobile phone shows that they are cooland connected. The explosions around theworld in mobile phone use make some health professionals worried. Some doctors are concerned that in the futuremany people may suffer health problems from the use of mobile phones. InEngland,there has been a seriousdebate about this issue. Mobile phone companies are worried about the negativepublicity of such ideas. They say that there is no proof that mobile phones arebad for your health. On the other hand,why dosome medical studies show changes in the brain cells of some people whouse mobile phones? Signs of change inthe tissues of the brain and:head can be detected with modern scanning(扫描)equipment In onecase,a traveling salesman had to retire at a young age because of seriousmemory loss. He couldnt remember evensimple tasks. He would often forget the name of his own son This man usedto talk on his mobile phone for aboutsix hours a day,every day of his working week,for a couple of years. His family doctor blamed his mobile phoneuse,but his employers doctor didnt agree. What is it that makes mobilephones potentially harmful? The answer is radiation High-tech machines candetect very small amounts of radiation from mobile phones. Mobile phonecompanies agree that there is some radiation,but they say the amount is toosmall to worry about. As the discussion abouttheir safety continues,it appears that its best to use mobile phones less often. Use your regular phone if you want totalk for a long time. Use your mobile phone only when you really need it. Mobile phones can be veryuseful and convenient,especiaIIY in emergencies. In the future,mobile phones may have a warning labelthat says they are bad for your health. So for now,its .wise not to use yourmobile phone too often.People buy cell phones for the following reasons EXCEPT thatA.they re cheapB.theyre popularC.theyre usefuD.theyre convenient 点击查看答案