The biggest safety threat facing airlines today may not be a terrorist with a gun, but the man with the portable computer in business class. In the last 15 years, pilots have reported well over 100 incidents that could have been caused by electromagnetic interference. The source of this interference remains unconfirmed, but increasingly, experts are pointing the blame at portable electronic device such as portable computers, radio and cassette players and mobile telephones.
　　RTCA, an organization which advises the aviation (航空) industry, has recommended that all airlines ban (禁止) 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. Currently, rules on using these devices are left up to individual airlines. And although some airlines prohibit passengers from using such equipment during take-off and landing, most are reluctant to enforce a total ban, given that many passengers want to work during flights.
　　The difficulty is predicting how electromagnetic fields might affect an aircraft’s computers. Experts know that portable device emit 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 vulnerable (易受损的) 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.

　　21. The passage is mainly about ________.
　　A) a new regulation for al airlines
　　B) the defects of electronic devices
　　C) a possible cause of aircraft crashes
　　D) effective safety measures for air flight（C）

　　22. What is said about the over 100 aircraft incidents in the past 15 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.（D）

　　23. Few airlines want to impose 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 refuse to take a plane which bans the use of radio and cassette players
　　D) they have other effective safety measures to fall back on（C）

　　24. 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.（C）

　　25. It can be inferred from the passage that the author ________.
　　A) is in favor of prohibiting passengers’ use of electronic devices completely
　　B) has overestimated the danger of electromagnetic interference
　　C) hasn’t formed his own opinion on this problem
　　D) regards it as unreasonable to exercise a total ban during flight（D）