Scientists have been
studying natural sources of methane for decades but hadn't regarded plants as a
producer, notes Frank Keppler, a geochemist at the Max Planck 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 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 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 Celsius. At 30 degrees Celsius, 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 methane per gram of plant tissue per hour. Methane emissions
tripled when living and dead plant was exposed 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. 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.
【正确答案】
C
【答案解析】[解析] 短文的第一段和第二段都讲到，科学家过去曾经认为，沼气必须在无氧的环境中才能产生。注意，作者用的是过去式：“most scientists assumed that...they had assumed that...”故选C。
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