题目内容：
Plant Gas Scientists have been studying natural sources of methane for decarlesd but hadn’t regarded plants as a produeer，notes Frank Keppler，a geochemist at the Max Planck Institute for Nuelear Physics in Heldelberg，Germany1 ． Now Keppler and his colleagues nnd that plants，from grasses to trees，may also be sources of the greenhouse gas． This is really surprising，hecause most scientists assumed that methane production requires an oxygen—free environment．Previously，researchers had thought that it was impossible for plants t0 make significant amounts of the gas． They had assumed that microhes2 need t0 he in environments without oxygen to prodnee methane． Methane is a greenhouse gas．1ike carbon dioxide． Gases sueh as methane and carhon dioxide trap heat in Earth’s atmosphere and contn‘hute to glohal warming．
In its experiments，Keppler’s team used sealed chambers that contained the same concentration of oxygen that Earth’s atmosphere has．They measured the amounts of methane that were released by both living plants and dried plant material，such as fallen leaves．
With the dried plants，the researchers took measurement at temperatures ranging from 30 degrees Celsius to 70 degrees C．At 30 degrees C，they found，a gram of dried plant material released up to
3 nanograms of methane per hour．One nanogram is a billionth of a gram．）With every 10-degree rise in temperature．the amount of methane released each hour roughly doubled．
Living plants growing at their normal temperatures released as much as 370 nanograms of meth’ane per gram of plant tissue per hour．Methane emissions tripled when living and dead plant was ex。posed to sunlight．
Because there was plenty of oxygen available，it’s unlikely that the types of bacteria that normally make methane were involved．Experiments on plants that were grown in water rather than soil also resulted in methane emissions．That’s another strong sign that the gas came from the plants and not soil microbes．
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 consum~’methane，they may prevent plant-produced methane from reaching the atmosphere．Field tests will be needed to assess the plant’s influence，she notes．What was scientists’understanding of methane? A．It was produced from plant
B．It was not a greenhouse ga
C．It was pmduced in OXygen－free environment
D．It traps more heat than any other greenhouse ga
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