单项选择题

民事法律行为依不同标准可划分为若干类型。关于民事法律行为类型的说法，正确的是（　）。

A.遗赠扶养协议属于单方法律行为
B.附义务的赠与属于有偿行为
C.租赁属于诺成行为
D.保证属于身份行为
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单项选择题
根据《行政处罚法》，下列有关行政处罚执行程序的说法中错误的是( )。
A.对于限制人身自由的行政处罚决定，只能由公安机关依法执行
B.作出罚款决定的机关与收缴罚款的机构分离，体现了职能分离原则
C.在法定的复议和诉讼期内，税务机关可以对处罚决定实施强制执行
D.当事人对行政处罚决定不服，申请行政复议或提起行政诉讼的除法律另有规定外，行政处罚不停止执行
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Plant GasScientists 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.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.
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