1)在开头出现;
2)在结尾出现;
3)在开头和结尾同时出现,首尾呼应;
4)有些文章的中心思想贯穿全文,并没有用一句话明确表达出来,这就要求学生学会归纳概括。
(三)实战演练
21 Astronomers have spent hundreds of years searching for signs of life on other planets, using telescopes. When they saw the geography of Mars , they thought they could see canals and that this might be evidence of intelligent life on the planet. More recently, however, spacecraft have been sent there to analyze the soil for signs of life. The result were negative and astronomers now feel certain that no life exist on the surface of any craft far beyond our solar system (太阳系)I not realistic because of the huge distances.
Unless life comes and visit us, the only way we likely to know of its existence is from radio message. Listening for intelligent extraterrestrial or E.T. life is not a new idea, but the techniques now being used offer a much better chance off success than before. Looking for evidence of life in other solar systems now means using special radio receivers called radio telescopes to listen out for message. The idea is that intelligent forms of life would have discovered radio waves and would, like us, be using them to communicate.
Through its research work, NASA has develop two means of systematically searching for E.T. life. The first method is to select the closest 800 stars like our sun and to direct a radio telescope to scan each of one them for electromagnetic wave. The second way is to survey the entire sky, listening for continuous signals which may come from extraterrestrial life. Scientists are looking in the 100-10,000 MegaHetz frequency rangea range where there are fewer natural radio signals which could interfere with transmission . By looking signals in these specific regions, scientists can avoid any confusion that might be caused by receiving a natural radio signal rather than one sent by intelligent life.
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