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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.  
What can we infer from the passage?
选项 A、Valley glaciers flow downhill from mountains and are shaped by the topography.
B、Ice sheets move faster than valley glaciers.
C、Valley glaciers are as old as ice sheets.
D、It takes different time for glaciers to form in different places.
答案D
解析 文章第三段主要论述的是冰川形成所需要的时间。在冰岛，降雪量大，夏季气温相对较高．这些都是加速冰川形成的条件。而在南极地区，降雪量小，终年酷寒，冰川形成较慢，因此可以推断在不同的地点形成冰川的时间是不一样的。
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