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根据下面材料，回答题。
A Thirsty World
The world is not only hungry, it is also thirsty for water. This may seem strange to you, since nearly 75% of the earth&39;s surface is covered with water. But about 97% of this huge amount is sea-water, or salt water. Man can only drink and use the other 3%--the fresh water that comes from rivers, lakes, underground, and other sources.____46____, because some of it is in the form. of icebergs and glaciers. Even worse, some of it has been polluted.
At the moment, his small amount of fresh water is still enough for us. However, our need for water is increasing rapidly. Only if we take steps to deal with this problem now, can we avoid a severe worldwide water shortage later on. One of the useful steps we can take is to stop unlimited use of water.____47____, however, would have a bad effect on agriculture and industry.
In addition to stopping wasting our precious water, one more useful step we should take is to develop ways of reusing it.____48____, but only on a small scale.
Today, in most large cities, water is used only once and it eventually returns to the sea or runs into underground storage tanks. But it is possible to pipe water that has been used to a purifying plant. There it can be filtered and treated with chemicals ____49____ .
____50____, we still would not have enough. Where could we turn next？ To the oceans! All we&39;d have to do to make use of the vast amount of sea-water is--remove the salt. This salt-removing process is already in use in many parts of the world. So if we take all these steps, we&39;ll be in no danger of drying up !
第46题__________
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The average modified mouse lived__________. 查看材料
膳食中存在的两种必需脂肪酸是()。
某人食入鲜牛奶后，出现胃肠不适，胀气、痉挛和腹泻的症状，一起进食的其他人则无此症状，该现象最可能的病因是()。
根据下面材料，回答题。
Eat More, Weigh Less, Live Longer
Clever genetic detective work may have found out the reason why a near-starvation diet prolongs the life of many animals.
Ronald Kahn at Harvard Medical School in Boston, U.S., and his colleagues have been able to extend the lifespan (寿命) of mice by 18 per cent by blocking the rodent&39;s (啮齿动物) increase of fat in specific cells. This suggests that thinness--and not necessarily diet--promotes long life in"calorie (热量卡) restricted" animals.
"It&39;s very cool work," says aging researcher Cynthia Kenyon of the University of California,San Francisco. "These mice eat all they want, lose weight and live longer. It&39;s like heaven."
Calorie restriction dramatically extends the lifespan of organisms as different as worms and rodents. Whether this works in humans is still unknown partly because few people are willing to submit to such a strict diet.
But many researchers hope they will be able to trigger the same effect with a drug once they understand how less food leads to a longer life. One theory is that eating less reduces the increase of harmful things that can damage cells. But Kahn&39;s team wondered whether the animals simply benefit by becoming thin.
To find out, they used biology tricks to disrupt the insulin (胰岛素) receptor (受体) gene in lab mice, but only in their fat cells. "Since insulin is needed to help fat cells store fat, these animals were protected against becoming fat," explains Kahn.
This slight genetic change in a single tissue had dramatic effects. By three months of age,Kahn&39;s modified mice had up to 70 per cent less body fat than normal control mice, despite the fact that they ate 55 per cent more food per gram of body weight.
In addition, their lifespan increased. The average control mouse lived 753 days, while the thin rodents averaged a lifespan of 887 days. After three years, all the control mice had died, but one-quarter of the modified rodents were still alive.
"That they get these effects by just manipulating the fat cells is controversial," says Leonard Guarente of the Massachusetts Institute of Technology, who studies calorie restriction and aging.
But Guarente says Kahn has yet to prove that the same effect is responsible for increased lifespan in calorie-restricted animals. "It might be the same effect or there might be two routes to long life," he points out,."and that would be very interesting."
Ronald Kahn and his colleagues can make mice live longer by__________. 查看材料
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