单选题
Scientists have been studying natural sources of methane (甲烷，沼气) for decades but hadn't regarded plants as a producer, notes Frank Keppler, a geochemist(地球化学家) at the Max Planek Institute for Nuclear Physics in Heidelberg, Germany. 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.
Previously, researchers had thought that it was impossible for plants to make significant amounts of the gas. They had assumed that microbes(微生物) need to be in environments without oxygen to produce methane. Methane is a greenhouse gas, like carbon dioxide. Gases such as methane and carbon dioxide trap heat in Earth's atmosphere and contribute to global warming.
In its experiments, Keppler's team used sealed chambers (室，房间；腔) 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(微克) 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 methane per gram of plant tissue per hour. Methane emissions tripled (增加三倍) when living and dead plant was exposed to sunlight.
Because there was plenty of oxygen available, it's unlikely that the types of bacteria( bacterium 的复数，细菌) 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(生物地球化学家) at the University of Minnesota in St. Paul. 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 plant's influence, she notes.

单选题 What was scientists' understanding of methane?
A. It was produced from plants.
B. It was not a greenhouse gas.
C. It was produced in oxygen-free environments.
D. It traps more heat than any other greenhouse gas.

单选题 To test whether plants are a source of methane, the scientists created ______.
A. a oxygen-free environment
B. an environment with the same concentration of oxygen as the Earth has
C. a carbon dioxide-free environment
D. an environment filled with the greenhouse gas

单选题 Which statement is true of the methane emissions of plants in the experiment?
A. The lower the temperature, the higher the amount of methane emissions.
B. Living plants releas less methane than dried plants at the same temperature.
C. When exposed to sunlight, plants stop releasing methane.
D. The higher the temperature, the greater the amount of methane emissions.

单选题 Which of the following about methane is Not mentioned in the passage?
A. Plants growing in soil release methane.
B. Plants growing in water release methane.
C. Soil microbes consume methane.
D. Microbes in plants produce methane.

单选题 What is the beneficial point of some microbes consuming plant-produced methane?
A. Methane becomes less poisonous.
B. Methane is turned into a fertilizer.
C. Less methane reaches the atmosphere.
D. Air becomes cleaner.