[单项选择]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 l
A. To show the temperature in the two places.
B. To show the environment in the two places.
C. To show the importance of temperature and the rate of snowfall in the formation of glaciers.
D. To show these two places contain the oldest glaciers in the worl

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