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. 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 expert lack equipment to do such research.
Belgium has three main geographic regions: the coastal plain, the central plateau, and the highlands. The coastal plain extends inland 16 to 48 kilometers on the northwest. Along the North Sea is a low-lying area consisting mainly of sandy hills and sections of land reclaimed from the sea. The coastal plain’s elevation ranges from sea level to 20 meters.
The central plateau is a gently rolling, slightly elevated area, irrigated by many waterways, and containing a number of wide fertile valleys with a rich soil. The highlands, a densely-wooded plateau averaging 460 meters in elevation, extend across southeastern Belgium and into northeastern France. Located here is the highest peak in Belgium with an elevation of 694 meters. The climate near the sea is humid and mild. Further inland, a marked increase in the range of temperature occurs. In the highlands, hot summers alternate with cold winters. Heavy rains are confined almost exclusively to the highlands. Fog and rai
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