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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.
We can conclude from this passage that ______.
选项 A、the heat created in microwave ovens is very difficult to control
B、food tastes better when cooked in a microwave oven
C、food cooked in microwave ovens is probably more nutritious than food cooked in more conventional ways
D、most research on the effects of microwaves on human beings has been restricted to the study of low level microwave radiation
答案D
解析 选项A和B在文中并未涉及；文章第二段最后一句指出"Food that have more water．．in a  microwave oven”，即对于在微波炉中烹饪的食物，所含水分越多，熟的越快，并且越有营养，而不是选项C所说的与传统的方法相比，在microwave ovens中烹饪的食物更有营养；至于选项D，文章第四段指出"scientists are more concerned about．．．a leaking microwave oven．”，即科学家们更加关心low level microwave radiation对人体的影响，故选项D正确。
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