题目内容
Space is a dangerous place, not only because of meteors (流星) but also because of rays from the sun and other stars. The atmosphere again acts【C1】______our protective blanket on【C2】______. Light gets through, and this is essential【C3】______plants to make the food which we【C4】______. Heat,【C5】______, makes our environments tolerable and some ultraviolet rays(紫外线) penetrate the【C6】______. Cosmic rays of various kinds come【C7】______the air from outer space, but【C8】______quantities of radiation from the sun are screened off.【C9】______ men leave the atmosphere they are【C10】______to this radiation;【C11】______their spacesuits or the walls of their spacecraft, if they are inside,【C12】______prevent a lot of radiation damage. Radiation is the greatest known danger to explorers in【C13】______. Doses of radiation are measured in【C14】______called "rems" (雷目). We all【C15】______radiation here on earth from the sun, from cosmic rays, and from radioactive minerals. The"【C16】______"dose of radiation that we receive each year is about two millirems; it【C17】______according to where you live, and this is a very rough estimate. Scientists have reason to think【C18】______a man can【C19】______far more radiation【C20】______without being damaged; the figure of 60 rems has been agreed.
【C1】
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听力原文： In ancient times, many people believed tile earth was a flat disc. Well over 2,000 years ago, rite ancient Greek philosophers were able to put forward two good arguments proving that it was not. Direct observations of heavenly bodies was the basis of both these arguments. First, the Greeks knew that during eclipses of the moon, file earth was between the sun and the moon, and they saw that during these eclipses, the earth's shadow on the moon was always round, they realized that this could be true only if the earth was spherical. If tile earth were a flat disc, then its shadow during eclipses would not be a perfect circle, it would be stretched out into a long ellipse. Tile second argument was based on what the Greeks saw during their travels. They noticed that the North Star, or Polaris, appeared lower in the sky when they traveled south, in more northern regions, the North Star appeared to them to be much higher in file sky. By the way, it was also from this difference in the apparent position of the North Star that the Greeks first calculated the approximate distance around the circumference of the earth, a figure recorded in ancient documents says 400,000 stadia, that's the plural of the word stadium. Today, it's not known exactly what length one stadium represents, but let's say, it was about 200 meters, the length of many athletic stadiums. This would hake the Greek' s estimate about twice the figure accepted today, a very good estimate for those writing so long before even the first telescope was invented.
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A．They noticed an apparent change in the position of the North Star.B．They observed ec
A．To suggest changes to government structure.B．To criticize the practice of slavery.C．
Section B
Directions: There are 2 passages in this section. Each passage is followed by some questions or unfinished statements. For each of them there are four choices marked A, B, C and D. You should decide on the best choice.
How do you send a message to a submerged submarine, particularly one carrying mi Water may not look like a barrier to communications, but appearances are deceiving. Water strongly absorbs all electro magnetic waves except blue green light and extremely low frequency radio waves. The very low frequency waves now used to contract submarines penetrates only a short distance into the ocean, so tile craft must either surface or send up and antenna(天线) to receive messages, thereby increasing its weakness. A la ser system—accurate over long distances and capable of carrying more data than tile very low frequency waves—would talk to submarine at their normal depths.
In the system, a very broad beam spreading out freely in all directions would be scanned(扫描) over thousands of square miles of ocean so that it wouldn't endanger boats, birds or fish—or the submarines it is supposed to reach.
Since only a small fraction of the laser system will make its way through the air and ocean, receivers mounted on the submarines must be able not only to detect the laser but also to discriminate between it and sunlight. So, military scientists are now working hard on special filter that allow through only the precise wavelengths emitted by the laser. The filtered light, when transformed into electrical signal, can then be decoded. Military planners are confident that laser communication with submarines is feasible.
Which of the following does the passage mainly discuss？
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