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  3. For all their great diversity of shapes and sizes, glaciers can be divided into two essential types: valley glaciers, which flow

For all their great diversity of shapes and sizes, glaciers can be divided into two essential types: valley glaciers, which flow

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问题      For all their great diversity of shapes and sizes, glaciers can be divided into two essential types: valley glaciers, which flow downhill from mountains and are shaped by the constraints of topography (地貌), and ice sheets, which flow outward in all directions from dome--like centers of accumulated ice to cover vast expanse of terrain. Whatever their type, most glaciers are remnants of great shrouds of ice that covered the earth years ago. In a few of these glaciers the oldest ice is very ancient indeed; the age of parts of the Antarctic sheet may exceed 500,000 years.
     Glaciers are born in rocky womb above the snow line, where there is sufficient winter snowfall and summer cold for snow to survive the annual melting. The long gestation period of a glacier begins with the accumulation and gradual transformation of snow flakes. Soon after they reach the ground, complex snowflakes are reduced to compact, roughly spherical ice crystals, and the basic components of a glacier. As new layers of snow and firm, snow that survives the melting of the previous summer, accumulate, they squeeze out most of the air bubbles trapped within and between the crystals below. This process of recrystallization continues throughout the life of the glacier.
     The length of time required for the creation of glacier ice depends mainly upon the temperature and the rate of snowfall. In Iceland, where snowfall is heavy and summer temperatures are high enough to produce plenty of melt--water, glacier ice may come into being in a relatively short time--say, ten years. In parts of Antarctica, where snowfall is scant and the ice remains well below its melting temperature year round, the process may require hundreds of years.
     The ice does not become a glacier until it moves under its own weight, and it cannot move significantly until it reaches a critical thickness--the point at which the weight of the piled-up layers overcomes the internal strength of the ice and the friction between the ice and the ground. This critica} thickness is about 60 feet. The fastest moving glaciers have been gauged at not much more than two and a half miles per year, and some cover less than 1/100 inch in that same amount of time. But no matter how infinitesimal the flow, movement is what distinguishes a glacier from a mere mass of ice.  
The author mainly describes ______ in the article.
选项 A、the creation of glaciers
B、the classification of glaciers
C、the features of glaciers
D、what glaciers are
答案A
解析 根据题干，定位到全文。文章共分四段．第一段主要论述冰川的分类，第二段主要论述冰川是怎样形成的；第三段主要论述冰川形成的时间差异以及造成这一差异的原因；第四段主要论述冰川形成的标志，因此可以推断作者主要描述了冰川的形成。
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