题目内容：

The biggest danger facing airlines nowadays may not be a terrorist with a gun, but the man with portable computer in business class. In the last 16 years, pilots have reported well  over 100 incidents that could have been caused by electromagnetic interference  (电磁干扰).  The source of this interference remains not proved, but increasingly, experts are pointing the  blame at portable electronic devices such as portable computers, radio and cassette players  and mobile telephones.

      RTC.A. an organization which advises the aviation  (航空 )industry, has suggested that all  airlines ban such devices from being used during "critical" stages of flight  particularly take-  off and landing. Some experts have gone further, calling for a total ban during all flights .Cur- rently , rules on using these devices are left up to individual airlines. And although some air- lines prevent passengers from using such equipment during take-off and landing, most are un- willing to carry out a total ban, given that many passengers want to work during flight.

     The difficulty is predicting how electromagnetic fields might affect an aircraft's computers. Experts know that portable devices send out radiation which affects those wavelengths which aircraft use for navigation and 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 fragile to interference raises the risk that terrorists may use radio systems in order to damage navigation equipment. As worrying, though, is the passenger who can 't hear the instructions to turn off his radio because the music's too loud.

1.What is said about the over 100 aircraft incidents in the past 16 years?

A. They may have been caused by the damage to-the radio systems.

B. They may have taken place during take-off and landing.

C. They were proved to have been caused by the passengers portable computers.

D. They were suspected to have resulted from electromagnetic interference.

2.Why is it difficult to predict the possible effects of electromagnetic fields on an airplane 's computers?

A. Because it is extremely dangerous to conduct such research on an airplane.

B. Because it remains a mystery what wavelengths are liable to be interfered with.

C. Because research scientists have not been able to produce the same effects in labs.

D. Because experts lack adequate equipment to do such research.

3.Few airlines want to perform a total ban on their passengers using electronic devices because_______

A. they don't believe there is such a danger as radio interference

B. the harmful effect of electromagnetic interference is yet to be proved

C. most passengers refused to take a plane ,which bans the use of radio and cassette players

D. they have other effective safety measures to fall back on

4.The passage is mainly about_________.

A. a new rule for all airlines

B. the disadvantages of electronic devices

C. a possible cause of aircraft incidents

D. effective safety measures for air flight