题干

如图是核反应堆的示意图,对于核反应堆的认识,下列说法正确的是(  )

A:铀棒是核燃料,核心物质是铀238

B:石墨起到吸收中子的作用

C:镉棒起到使中子减速的作用

D:水泥防护层的作用是为了阻隔γ射线,避免放射性危害

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D

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       According to a new scientific research, we are rapidly going close to major climate change and the effects on the environment could be quite serious. Geographers predict that climate zones could completely disappear within the next eighty years. Where will crops be grown if this happens? When the climate changes a lot, polar regions will get colder and tropical regions will get even hotter. As a result, where will animals be forced to move on the earth?
       Climate changes like these could lead to the spread of diseases. Tropical storms and hurricanes(飓风) will not only increase but may also become stronger. If the changes come too quickly, animals and plants may not be able to adapt(适应) right away and could disappear from the earth.
       According to Science Daily, a new study predicts that, by the year 2100, many of today's familiar climates will be taken place by climate unknown in today's world. It is urgent that we reduce the risks of far-reaching results for the whole world. The planet itself has been showing signs of change. In 2004, a huge tsunami created by a major earthquake killed thousands in Sumatra and in 2008, thousands more died in China in another big earthquake. Egypt was hit in 2009 with a major earthquake and Haiti was destroyed seriously and became ruins in 2010 by another big earthquake.
       Not long ago, scientists discovered ice melting(融化) in west Antarctica(南极洲). This is very worrying because temperatures in this area have remained rather constant(不断的) and actually even lower at time. Melting occurs when temperatures increase and since temperatures have remained, the ice should not be melting. However, satellite data(数据) shows melting around west Antarctica.
       Within just the last few months, new reports from around the world have been coming in and most agree that our climate situation is much worse than previously expected. At this point, it doesn't matter what is causing it, but rather, what can be done about it. On the other hand, our world is getting more and more unstable(不稳定) every year. Natural disasters(灾难) are becoming more frequent(频繁的) and serious.
       However, other planets are experiencing global warming as well as our own and some scientists believe there may be some connections between these. No one knows anything for sure at this point because there is simply not enough data.