2. 外语
  3. Ecologists had assumed that trees in the consistently warm tropics grew at a slow but steady rate, unvarying from year to year.

Ecologists had assumed that trees in the consistently warm tropics grew at a slow but steady rate, unvarying from year to year.

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问题     Ecologists had assumed that trees in the consistently warm tropics grew at a slow but steady rate, unvarying from year to year. However, a study at La Selva, Costa Rica, showed that trees grew less in hotter years and more in cooler ones: between 1984 and 2000, dramatic differences occurred in the six species of trees studied, with trees adding twice as much wood in some cooler years as they did in the scorching El Nino year of 1997-1998. Because tree growth is an index of the balance between photosynthesis, in which trees absorb carbon dioxide (CO2) from the atmosphere and release oxygen, and respiration, in which the opposite occurs, the La Selva data were the first hint that rapidly rising global temperatures, driven by human-generated emissions of CO2, may be pushing tropical forests to release more CO2, thereby intensifying global warming. This raised serious questions about a popular theory that tropical forests act as a sponge, soaking up much of the excess CO2 that humans pump into the atmosphere. The La Selva data are consistent with a model of global CO2 flux developed by Keeling, who concluded that the amount of CO2 taken up in tropical landmasses rose in cooler years and fell in hotter ones, accounting for year-to-year changes in the amount of CO2 that stays in the atmosphere.
The passage suggests that as temperatures rise, trees in the tropical regions
选项 A、continue to grow at a slow but steady rate
B、grow less and intensify photosynthesis
C、emit more CO2 and oxygen
D、increase overall respiration and decrease overall photosynthesis
E、grow more and absorb more CO2
答案D
解析 GRE阅读特别喜欢考查“事件链”，尤其是理工科文章中的生物类和环境类，比如本文的“热=厄尔尼诺=长得慢=呼吸作用强(因为呼吸消耗能量，这会削弱树木的增长)=CO2上升(呼吸作用释放CO2)”这样几个环节之间的相互作用。对于生物类文章，通常以食物链的形式展现各个环节之间的关系，比如A动物多，导致B动物少，又导致C动物多，D动物少……对于环境类，主要考查自然环境中各个部分的相互作用，比如这篇文章考查的便是温度、CO2、树木增长等方面的关联。建议考生考试的时候如果遇到“事件链”，请务必做好笔记， “事件链”通常会出题。
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