2. 考研
  3. In the following text, some sentences have been removed. For Questions 41-45, choose the most suitable one from the list (A、B、C、

In the following text, some sentences have been removed. For Questions 41-45, choose the most suitable one from the list (A、B、C、

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问题 In the following text, some sentences have been removed. For Questions 41-45, choose the most suitable one from the list (A、B、C、D、E、F、G……) to fit into each of the numbered blank. There are several extra choices, which do not fit in any of the gaps. (10 points)

    The atmosphere is a mixture of several gases. There are about ten chemical elements which remain permanently in gaseous form in the atmosphere under all natural conditions. Of these permanent gases, oxygen makes up about 21 percent and nitrogen about 78 percent. (41)______. The amount of water vapor, and its variations in amount and distribution is of extraordinary importance in weather changes. Atmospheric gases hold in suspension great quantities of dust, pollen, smoke, and other impurities which are always present in considerable, but variable amounts.
    The atmosphere has no definite upper limits but gradually thins until it becomes imperceptible(难以察觉的). Until recently it was assumed that the air above the first few miles gradually grew thinner and colder at a constant rate. (42)______. Recent studies of the upper atmosphere, currently being conducted by earth satellites and missile probing(探查), have shown these assumptions to be incorrect. The atmosphere has three well-defined strata(层).
    The layer of the air next to the earth, which extends upward for about ten miles, is known as the troposphere(对流层). On the whole, it makes up about 75 percent of all the weight of the atmosphere. (43)______. A steady decrease of temperature with increasing elevation is a most striking characteristic. The upper layers are colder because of their greater distance from the earth’s surface and rapid radiation of heat into space. The temperatures within the troposphere de crease about 3.5 degrees per 1,000 feet increase in altitude. Within the troposphere, winds and air currents distribute heat and moisture. Strong winds, called jet streams, are located at the upper levels of the troposphere. These jet streams are both complex and widespread in occurrence. They normally show a wave shaped pattern and move from west to east at velocities of 150 mph, but velocities as high as 400 mph have been noted. The influences of changing locations and strengths of jet streams upon weather conditions and patterns are no doubt considerable. Current intensive research may eventually reveal their true significance.
    (44)______. The stratosphere is separated from the troposphere by a zone of uniform temperatures called the tropopause. Within the lower portion of the stratosphere is a layer of ozone(臭氧) gases which filters out most of the ultraviolet rays from the sun. The ozone layer varies with air pressure. If this zone were not there, the full blast of the sun’s ultraviolet light would burn our skins, blind our eyes, and eventually result in our destruction. Within the stratosphere, the temperature and atmospheric composition are relatively uniform.
    The layer upward of about 50 miles is the most fascinating but the least known of the three strata. (45)______. The northern lights (aurora borealis) originates within this highly charged portion of the atmosphere, as effect upon weather conditions if any, is as yet, unknown.

A. It is the warmest part of the atmosphere because most of the solar radiation is absorbed by the earth’s surface which warms the air immediately, surrounding it.
B. It is hard to define the three layer of the atmosphere.
C. Several other gases, such as argon, carbon dioxide, hydrogen, neon, krypton, and xenon, comprise the remaining one percent of the volume of dry air
D. Nowadays, the atmosphere is polluted by mankind.
E. It is called the ionosphere because it consists of electrically charged particles called ions, thrown from the sun.
F. It was also assumed that upper air had little influence on weather changes.
G. Above the troposphere to a height of about 50 miles is a zone called the stratosphere.

选项
答案G
解析 按照本文的结构来看，连续三段的内容是分别介绍对流层、平流层和电离层的，那么本段重点就是讲述平流层的特点。作为本段的第一句话，选项G是最合适的。
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