根据以下材料，回答下面试题
　　Headphones used with MP3 digital music players like the iPod may interfere with heart pacemakers (起搏器 ) and implantable defibrillators ( 除颤器 ), U.S. researchers said.
　　The MP3 players themselves posed no threat to pacemakers and defibrillators, used to normalize heart rhythm. But strong little magnets inside the headphones can confuse the devices if placed within 1.2 inches of them, the researchers told an American Heart Association meeting in New Orleans.
　　Dr. William Maisel of the Medical Device Safety Institute in Boston led a team that tested eight models of MP3 player headphones in 60 defibrillator and pacemaker patients.
　　They placed the headphones on the patients' chests, directly over the devices. The headphones interfered with the heart devices in about a quarter of the patients-14 of the 60-and interference was twice as likely in those with a defibrillator than with a pacemaker.
　　Another study presented at the meeting showed that cell phones equipped with wireless technology are unlikely to interfere with pacemakers.
　　A pacemaker sends electrical impulses to the heart to speed up or slow heart rhythm. The magnet, however, could make it deliver a signal no matter what the heart rate is, the researchers said.
　　An implantable defibrillator signals the heart to normalize its rhythm if it gets too fast or slow. A magnet could de-activate it, making it ignore an abnormal heart rhythm instead of delivering an electrical shock to normalize it.
　　The devices usually go back to working the right way after the headphones are removed,the researchers said.
　　"The main message here is:it's fine for patients to use their headphones normally,meaning they can listen to music and keep the headphones in their ears. But what they should not do is put the headphones near their device," Maisel said in a telephone interview.
　　So that means people with pacemakers or defibrillators should not place the headphones in a shirt pocket or coat pocket near the chest when they are not being used, and should not place them over their chest or have others who are wearing headphones rest their head on the
　　patient's chest, Maisel said.
　　第21题 How can MP3 digital music players hinder pacemakers and defibrillators?
　　A.MP3 players can interfere with heart pacemakers and defibrillators.
　　B.The magnets inside the headphones can interfere with pacemakers and defibrillators.
　　C.The loud music beats pose a threat to pacemakers and defibrillators.
　　D.MP3 players are placed too close to pacemakers and defibrillators.
　　第22题 Dr. William Maisel's tests showed that_____.
　　A.headphones had interference with the heart devices in everypatient
　　B.haft of the models of MP3 player headphones had interferencewith heart devices
　　C.headphones had much stronger interference with adefibrillator than with a pacemaker
　　D.headphones had much stronger interference with a pacemakerthan with a defibrillator
　　第23题 The magnets inside the headphones can cause problemsby_____.
　　A.sending out electrical shock to damage hearts
　　B.sending out signals to make hearts beat too slow
　　C.sending out signals to make hearts beat too fast
　　D.making the heart devices malfunction
　　第24题 People with pacemakers or defibrillators should_____.
　　A.never use MP3 digital music players
　　B.not use MP3 headphones
　　C.not use the headphones near their hearts
　　D.put the headphones in a pocket when they are not being used
　　第25题 The writer's purpose in writing this article is to_____.
　　A.report a scientific research
　　B.warn people not to use modern gadgets
　　C.compare different headphone products
　　D.inform people of the safe use of MP3 players

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