[单项选择]

Questions 15 to 19 are based on the following passage.
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 devices 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
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