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  3. Microwaves are a type of electromagnetic radiation; they are a very mild form of electrical or magnetic wave that moves through

Microwaves are a type of electromagnetic radiation; they are a very mild form of electrical or magnetic wave that moves through

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问题     Microwaves are a type of electromagnetic radiation; they are a very mild form of electrical or magnetic wave that moves through space. Unlike X-rays and gamma rays, which are very powerful rays of radiation, microwaves are rather weak and are much more like the waves of radiation used in radio broadcasting.
    In microwave ovens, the use of microwaves with which most people me familiar, the waves are produced by an electronic tube called a magnetron. Microwaves produce heat in any food placed inside  the oven by causing the water in the food to vibrate rapidly and thus heat up. Food that have more water in them take less time to cook and probably have more of their nutrients left intact when cooked in a microwave oven.
    Microwaves do not pass through metal, so the microwaves are retained within the oven. Microwaves  pass immediately through glass, paper, and plastic with no effect on these materials or on the microwaves; nothing inside the microwave oven is heated except the food itself, so cooking process is much more efficient than in conventional ovens. Sometimes a pan or container is heated because it is touching the hot food, though: some users of microwave oven have been burned by but food, by hot pans, or steam escaping from the food. No documented case of radiation burns born a microwave oven has ever been reported.
    Actually, we know very little about how microwave radiation might affect human beings. Obviously,  if microwave can cook a roast by exciting the water molecules in the meat, they could do the same thing to human flesh. Human being could be burned by prolonged exposure to high levels of microwaves.   But scientists are more concerned about the effects of low level microwave exposures, such as might result from a leaking microwave oven.
    No research has yet been performed on people who have been exposed to low level microwave radiation. Some experiments have been performed on animals, but the results are very difficult to interpret.  As the eyes are particularly sensitive, rabbits exposed to low level microwaves were checked for the growth of cataracts, and none were found. On the other hand, some animals seem able to sense microwave radiation and try to escape from it immediately. In others, microwave radiation causes  the body to react as if defending itself against a decease. These responses lead some scientists to think that microwave radiation is harmful, though in some yet undiscovered way.
As used in the 3ut paragraph of this passage, the word "documnted" means ______.
选项 A、numbered
B、recorded
C、legal
D、believable
答案B
解析 根据本句中的"case"和"report"判断，选项B正确。documented备有证明文件的。recorded有记录的。numbered有限的，时日无多的。legal法律的，法定的，合法的。 believable可信的。
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