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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 studies, 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 supports which of the following statements about the trees in the La Selva study?

ADuring the El Nino year, they added considerably less wood than they did in cooler years during the period of 1984-2000. 
BDuring the El Nino year, they typically had higher rates of photosynthesis than they did in other years during the period of 1984-2000. 
CDuring the El Nino year, they released considerably more oxygen than they did in cooler years during the period of 1984-2000. 
DDuring the El Nino year, they took up considerably more CO2 than they did in cooler years during the period of 1984-2000. 
EThe amount of CO2 that they absorbed remained constant throughout the entire period of 1984-2000.

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