The biggest safety threat（威胁）facing airlines today may not be a man with a gun ,but the man with portable（便携式）computer in business class .In the past 15 years ,pilots have reported about over 100 incidents that could have been caused by electro-magnetic interference(电磁干扰) .The source of this interference remain unclear ,but increasingly ,experts are pointing the blame at portable electronic devices（设备）such as portable computers ,radio and mobile telephones .
RTCA ,an organization which advises the aviation industry ,has suggested that all airlines ban, such devices from being used during “key” stages of flight ,particularly take-off and landing .Some experts have gone further ,calling for a total ban during all flights .Nowadays ,rules on using these devices are left up to airlines. And although some airlines ban passengers from using such equipment during take-off and landing ,most are not willing to exercise a total ban ,if many passengers want to work during flights .
The difficulty is understanding how electromagnetic fields might affect an aircraft’s computers .Experts know that portable devices give off radiation which affects those wave lengths which aircraft uses for communication .But because they have not been able to reproduce these effects in a laboratory ,they have no way of knowing whether the interference might be dangerous or not .
The fact that aircraft may be in trouble because of the interference raises the danger
that some people may use radio systems in order to damage navigation（导航）equipment .As worrying ,though ,is the passenger who can’t hear the instruction to turn off his radio because the music is too loud .
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.What is said about over 100 aircraft incidents in the past 15 years ?
A They may have taken place during take-off and landing .
B They have been caused by the damage to the radio systems .
C They were said to have resulted from electro-magnetic interference .
D They were caused by the passengers’ portable computers .
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. Few airlines want to refuse a total ban on their passengers because ____ .
A they have other effective safety measures to fall back on
B they don’t believe there is such a danger as radio interference
C the effect of electromagnetic interference is yet to be proved
D most passengers refuse to take a plane which bans the use of radio 
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. Why is it difficult to see the effects of electromagnetic fields on computers ?
A Experts lack proper equipment to such research .
B It is dangerous to carry out such research on an airplane .
C It remains a question what wave lengths are to be interfered with .
D Scientists are not able to produce the same effects in laboratory .
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It can be inferred from the passage that the author ___ .
A hasn’t formed his own opinion on this problem
B has fully understood the danger of electromagnetic interference
C regards it is as unreasonable to exercise a total ban during flight
D is in favor of banning passengers’ use of electronic devices