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  3. Changes in regional climate brought about by large-scale deforestation in the eastern lowlands of Central America are affecting

Changes in regional climate brought about by large-scale deforestation in the eastern lowlands of Central America are affecting

admin2010-07-06  48
问题      Changes in regional climate brought about by large-scale deforestation in the eastern lowlands of Central America are affecting weather downwind in the mountains, imperiling ecosystems there.
     The so-called cloud forests of Monteverde lie along the crest of Costa Rica’s Cordillera de Tilaran mountains. These habitats rely on the almost perpetual fog that forms as moisture-laden Caribbean winds rise up the eastern slopes of the mountains and pass through altitudes at which clouds condense, says Robert O. Lawton, an ecologist at the University of Alabama in Huntsville. The humidity in those breezes is enhanced by moisture expelled from the leaves of lowland forests.
     By the early 1990s, more than a century of deforestation had left only 18% of the Costa Rican lowland forests east of the peaks untouched.  The pastures that replaced forests don’t humidify the winds as well as forests do and are better at warming the atmosphere. As a result, the winds off these pastures must rise further up the Cordillera de Tilaran slopes before clouds condense.
    Satellite photos of the lowlands in the dry season show that clouds are absent or sparse over deforested areas but are thicker over the forests of neighboring Nicaragua. Computer simulations of day-time cloud formation in the area support these observations, Lawton notes, and they also suggest that the altitude of the cloud base would rise about 200m above today’s height if the lowlands were completely deforested.  Lawton and his colleagues report their result in Science magazine.
     The gradual shifting of bird ranges upslope and a recent population crash among frogs and toads in the Monteverde cloud forest suggest that the veil of clouds may be lifting. Scientists had already blamed the rise of the cloud base for the longer periods of mistfree conditions observed at the downwind edge of the forest. Lawton warns that in the future, the clouds may disappear from the Monteverde slopes for days at a time during the dry season -- a development that could lead to collapse of the ecosystems there.
Which of the following words can replace the word "as" in the second paragraph?
选项 A、which
B、that
C、when
D、while
答案C
解析 第二段的第二句话中的as相当于when，这句话谈到了这些栖息地赖以生存的源源不断的雾是在什么情况下形成的；as引出的句子在that从句中作时间状语从句。
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