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进行概率分析时，（）是用来描述随机变量的一个主要参数。 A．期望值 B．净现值 C．平均值 D．净年值
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总监理工程师由监理人任命，发包人应在（）将总监理工程师的任命通知承包人。A．发出开工通知前B．发出开工通知后C．递交申请施工许可证前D．颁发施工许可证后
The United States is trying to ________ the problems created by the energy crisis.
Questions 26 to 30 are based on the following passage. It is often claimed that nuclear energy is something we cannot do without. We live in a consumer society where there is an enormous demand for commercial products of all kinds. Moreover, an increase in industrial production is considered to be one solution to the problem of mass unemployment. Such an increase presumes an abundant and cheap energy supply. Many people believe that nuclear energy provides an inexhaustible and economical source of power and that it is therefore essential for an industrially developing society. There are a number of other advantages in the use of nuclear energy. Firstly, nuclear power, except for accidents, is clean. A further advantage is that a nuclear power station can be run and maintained by relatively few technical and administrative staff. The nuclear reactor represents an enormous step in our scientific evolution and, whatever the anti nuclear group says, it is wrong to expect a return to more primitive sources of fuel. However, opponents of nuclear energy point out that nuclear power stations bring a direct threat not only to the environment but also to civil liberties.
Furthermore, it is questionable whether ultimately nuclear power is a cheap source of energy. There have, for example, been very costly accidents in America, in Britain and, of course, in Russia. The possibility of increases in the cost of uranium (铀) in addition to the cost of greater safety provisions could price nuclear power out of the market. In the long run, environmentalists argue, nuclear energy wastes valuable resources and disturbs the ecology to an extent which could bring about the destruction of the human race. Thus, if we wish to survive, we cannot afford nuclear energy. In spite of the case against nuclear energy outlined above, nuclear energy programmes are expanding. Such an expansion assumes a continual growth in industrial production and consumer demands. However, it is doubtful whether this growth will or can continue. Having weighed up the arguments on both sides, it seems there are good economic and ecological reasons for sources of energy other than nuclear power.
第26题：The writer’s attitude toward nuclear energy is ________.
Questions 26 to 30 are based on the following passage. There are some each phenomena you can count on, but the magnetic field, someday is not of them. It fluctuates in strength, drifts from its axis, and every few 100,000 years undergo, dramatic polarity reversal-a period when North Pole becomes South Pole and South Pole becomes North Pole. But how is the field generated, and why is it so unstable？
Groundbreaking research by two French geophysicists promises to shed some light on the mystery. Using 80 metres of deep sea sediment (沉淀物) core, they have obtained measurements lots of magnetic-field intensity that span 11 polarity reversals and four million years. The analysis reveals that intensity appears to fluctuate with a clear, well-defined rhythm. Although the strength of the magnetic field varies irregularly during the short term, there seems to be an inevitable long-term decline preceding each polarity reversal. When the poles flip—a process that takes several hundred thousand years-the magnetic field rapidly regains its strength and the cycle is repeated.
The results have caused a stir among geophysicists. The magnetic field is thought to originate from molten (熔化的) iron in the outer core, 3,000 kilometers beneath the earth’s surface. By studying mineral grains found in material ranging from rocks to clay articles, previous researchers have already been able to identify reversals dating back 170 million years, including the most recent switch 730,000 years ago. How and why they occur, however, has been widely debated. Several theories link polarity flips to external disasters such as meteor (陨星) impacts. But Peter Olson, a geophysicist at the Johns Hopkins University in Baltimore, says this is unlikely if the French researchers are right. In fact, Olson says intensity that predictably declines from one reversal to the next contradicts 90 percent of the models currently under study. If the results prove to be valid geophysicists will have a new theory to guide them in their quest to understand the earth’s inner physics. It certainly points the direction for future research.
第26题：Which of the following titles is most appropriate to the passage？
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