The radio waves used for radar are very short，only（）long.

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• 翻译：“Since sound waves are carried by motion of matter, there must be matter around for the waves to travel through.”

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• 听力原文：Italian scientists have raised new health concerns about the safety of using mobile phones, with research showing radio waves from the handsets makes cancerous cells grow more aggressively. When Fiorenzo Marinelli and his colleagues at the National Research Council in Bologna exposed leukemia cells in the laboratory to 48 hours of continuous radio waves they initially killed the cancer cells but then made the surviving tumor cells replicate more rapidly.

  We don't know what the effects would be on healthy human cells, Marinelli told New Scientist magazine. In the Italian study, after 24 hours 20 percent more leukemia cells died than healthy cells but longer exposure to the radio waves triggered genes in the surviving cancer cells to divide aggressively. The results of the study do not show any direct threat to human health but they support the belief of some scientists who say radiation can damage DNA and destroy the cell repair system, thus making tumors more deadly. But animal studies, including recent research by Australian scientists at the Institute of Medical and Veterinary Science in Adelaide, have shown that radiation from mobile phones does not trigger the growth of tumors. The WHO (i.e., The World Health Organization) has called for more research into the potential health hazards of mobile phones and has urged people to limit their use of them. A British government inquiry, which concluded that there was no evidence that mobile phones are a danger to health, has advised parents to discourage their children, whose brains are still developing, from using them excessively. What is the speaker's main topic？ A．Effects of using mobile phones on healthy human cells. B．Risks of developing cancer involved in the use of mobile phones. C．Damage to healthy cells caused by the use of mobile phones. D．The potential health hazards of mobile phones on humans.

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• The basic principle of radar is illustrated by what happens when one shouts in a e. The echoof the shouting 11 the walls helps a person determine the size of i cave. With radar, however , the waves are radio waves instead of 12 waves. dio waves travel 13 the speed of light, about 300,000 kilometers in a second. A lar set 14 a short burst of radio waves. Then it receives the echoes 15 when waves bounce (Silt) off objects. By determining the time it 16 for the echoes to imi to the radar set, a trained technician 17 determine the distance between the [ar set and other objects. The word "radar", 18 , gets its name from the term dio detection and ranging." "Ranging" is the term for detection of the 19 between object and the radar set.

  Besides being of critical importance to pilot, radar is 20 for air traffic control, tracking ships at sea, and for tracking weather systems and storms. 11. A.through B.for C.on D.against

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