单选题 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.

单选题 As used in the 3ut paragraph of this passage, the word "documnted" means ______.
A. numbered B. recorded C. legal D. believable

单选题 The purpose of this passage is to discuss ______.
A. the beneficial use of microwaves
B. the harmful effects of microwaves
C. the modern methods of cooking
D. the use and possible harmful effects of microwaves

单选题 Microwaves are ______.
A. absolutely safe at low levels
B. very efficient for cooking
C. cancer-causing iii people with weak eyes
D. completely understood by scientists

单选题 We can conclude from this passage that ______.
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

单选题 The author' s tone is ______.
A. objective B. sarcastic C. negative D. skeptical