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A．Because they have a smooth surface.B．Because they are very hard.C．Because they are v
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听力原文：W: Air pollution is one of the most serious problems in the world. Polluted air can make us sick or even kill us. Why isn't more done to stop it？
M: Everyone wants to stop it, but it is a difficult problem to solve. Meat pollution is mused by things people need.
W: What do you mean？
M: Well, automobiles and airplanes cause pollution but they also provide transportation.
W: Right. And factories cause air pollution but they provide people with jobs and products.
M: Cities are the worst offenders. In crowded cities, factories and cars can add tons of pollution to the air every day.
W: Yes. When I drove, to town yesterday, the clear air changed. It seemed like a dark cloud hung over the city.
M: Nobody likes air pollution. It smells bad; it makes it difficult to breathe.
W: True, it even damages the plants that provide tis with food. No wonder we get sick. It's dangerous to everyone's health.
M: I think everyone knows it is a serious problem. When air pollution can even damage strong materials like steel and concrete; think what it does to poor humans.
W: It is terrible, and yet we do need some things that cause it. We'll have to try to figure out a solution.
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A．A crowded city.B．A busy airport.C．A steel factory.D．An old car park.
A．One second.B．Two years.C．One minute.D．A day.
听力原文： A computer is a machine designed to perform. work mathematically and to store and select information that has been fed into it. It is run by either mechanical or electronic means. These machines can do a great deal of complicated work in a very short time. A large computer, for example, can add or subtract nine thousand times a second, multiply a thousand times a second, or divide five hundred times a second. Its percentage of error is about one in a billion digits. It has been estimated that human beings making calculations average about one mistake per two hundred digits.
The heart of an electronic computer lies in its vacuum tubes, or transistors. Its electronic circuits work a thousand times faster than the nerve cells in the human brain. A problem that might take a human being two years to solve can be solved by a computer in one minute, but in order to work properly, a computer must be given instructions--it must be programmed.
Computers can be designed for many specialized purposes--they can be used to prepare payrolls, guide airplane flights, direct traffic, even to play chess. Computers play an essential role in modern automation in many plants and factories throughout the world.
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