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听力原文：M: Doesn't your class start at 8:00 every morning？
W: Not really. On Wednesdays it doesn't start until 8:30 and on Thursdays not until 10:00.
Q: What time does the woman's class start on Tuesdays？
(19)
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A．To let the children correct their own papers.B．To let the children measure their own
听力原文： Have you ever felt your mind got confused after a sleepless night？ You couldn't come up with an original thought no matter how hard you tried.
You were probably right if you thought that was caused by a lack of sleep. Dr. Home, a sleep researcher in England, studied 24 college students divided into two groups. One group got their normal eight hours of sleep. The other group didn't get the smallest amount of sleep--they stayed awake all night. The next day, Dr. Home tested the students. He asked them questions that required creative and original thinking. One of the questions was "How many uses can a cardboard box be put to？"
The results？ The wide-awake students did well on the tests. The tired students did poorly.
Research has already shown that tired people can do okay on tests of routine thinking, like simple addition. But Dr. Home tested creative thinking only.
As part of his study, he offered an amount of money as a reward to the sleepy students if they did well. But even this encouragement wasn't enough to help the students overcome their tiredness. They still did poorly. Dr. Home believes that the cerebral cortex--the part of the brain where thinking takes place--may get worn out during waking hours. Sleep may help to repair the brain overnight. "Without any sleep," he emphasizes, "even if you concentrate harder, you cannot do better." This study gives people something to think about, especially people like hospital workers and airline pilots, who must stay awake all night and then make emergency decisions.
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The medical expenses of the aged are 6 times more than the costs for the middle-aged.
听力原文：W: Dr. Cottam. You're studying the effects of air pollution on trees？
M" That's right.
W: Could you tell us something about it？
M: Trees in industrial areas grow much more slowly than they should. Worse still, trees hundreds of miles away also seem to be damaged by these same industries.
W: You mean "acid rain", I suppose.
M: Yes.
W: What effects does acid rain have on the environment'？
M: For a start, lakes polluted by it in this way can no longer support any fish. Gradually the acid rain washes elements out of the soil essential for healthy plant growth.
W: But I think that there's no general agreement on the causes and effects of acid rain.
M: That's right. The distances involved are so great that it's impossible to prove what's actually going on. And no one will willingly accept even partial responsibility for something that happens miles away.
W: But trees are dying. No one can deny that.
M: Exactly.
W: So how does your investigation go？
M: We've set up special glasshouses to keep a careful observation on the growth of the plants. We pump in controlled amounts of air pollutants, including ozone, which isn't always associated with acid rain.
W: Which industries emit ozone？
M: Mainly oil refining industry. We also occasionally cool the atmosphere in the greenhouses, which causes frost.
W: Have you had any results from these experiments yet？
M: It's still too early to draw any definite conclusions. But we've noticed that plants at low temperatures are affected more severely than those at normal temperatures.
W: So cold weather makes the effects of pollution worse？
M: It certainly looks that way. But we still hope to be able to persuade industries to take measures to reduce emissions of gases into the atmosphere.
(20)
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