【单选题】

Glaciers are large masses of ice on land that show evidence of past or present　　　 movement. They grow by the gradual transformation of snow into glacier ice.　　　

A、fresh snowfall is a fluffy mass of loosely packed snowflakes, small delicate ice　　　 constals grown in the atmosphere.As the snow ages on the ground for weeks or months,(5) the crystals shrink and become more compact, and the whole mass becomes squeezed　　　 together into a more dense form, granular snow.As new snow falls and buries the older　　　 snow, the layers of granular snow further compact to form firm, a much denser kind of　　　 snow, usually a year or more old, which has little pore space. Further burial and slow　　　 cementation―a process by which crystals become bound together in a mosaic of(10)intergrown ice crystals―finally produce solid glacial ice. In this process of　　　 recrystallization, the growth of new crystals at the expense of old ones, the percentage of　　　 air is reduced from about 90 percent for snowflakes to less than 20 percent for glacier ice.　　　 The whole process may take as little as a few years, but more likely ten or twenty years or　　　 longer. The snow is usually many meters deep by the time the lower layers art convened(15)into ice.　　　 In cold glaciers those formed in the coldest regions of theEarth, the entire mass of ice　　　 is at temperatures below the melting point and no free water exists. In temperate glaciers,　　　 the ice is at the melting point at every pressure level within the glacier, and free water is　　　 present as small drops or as larger accumulations in tunnels within or beneath the ice.(20)Formation of a glacier is complete when ice has accumulated to a thickness (and thus　　　 weight) sufficient to make it move slowly under pressure, in much the same way that solid　　　 rock deep within theEarth can change shape without breaking. Once that point is reached,　　　 the ice flows downhill, either as a tongue of ice filling a valley or as thick ice cap that　　　 flows out in directions from the highest central area where the most snow accumulates.　　　 The up down leads to the eventual melting of ice.It can be inferred from the last paragraph that a glacier
A、can revert to a fluffy mass
B．maintains the same shape through out the glacial process
C．is too cold to be thoroughly studied
D．can contribute water to lakes, rivers, or oceans