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阅读下面的短文。每篇短文的后面有五个问题，每个问题有四个备选答案。请根据短文的内容选择最佳答案。每个试题计3分,共计15分。 Scientists have been studying natural sources of methane for decades but hadnt regarded plants as a producer，notes Frank Keppler，a geochem[st at the Max Planck Institute for Nuclear Physics in Heidelberg，Germany1.Now Keppler and his colleagues find that plants，from grasses to trees，may also be sources of the greenhouse gas.This is really surprising，because most scientists assumed that methane production requires an oxygen-free environment. In its experiments，Kepplers team used sealed chambers that contained the same concentration of oxygen that Earths atmosphere has.They measured the amounts of methane that were released by both living plants;and dried plant material，such as fallen leaves. Living plants growing at their normal temperatures released as much as 370 nanograms of methane per gram of plant tissue per hour.Methane emissions tripled when living and dead plant was exposed to sunlight. The new finding is an interesting observation, says Jennifer Y.King，a biogeochemist at the University of Minnesota in St.Paul3.Because some types of soil microbes consume methane，they may prevent plant-produced methane from reaching the atmosphere.Field tests will be needed to assess the plants influence，she notes. | 
