“Fourteen percent of emissions from all sources in Australia are from enteric methane from cattle and sheep，” said Athol Klieve，a senior research scientist with the Queensland state government.

　　“And if you look at another country such as New Zealand，which has got a much higher agricultural base，they're actually up around 50 percent，” he said.

　　Researchers say the bacteria also make the digestive process much more efficient and could potentially save millions of dollars in feed costs for farmers.

　　But it will take researchers at least three years to isolate the bacteria，before they can even start to develop a way of transferring it to cattle and sheep.

　　Another group of scientists，meanwhile，has suggested Australians should farm fewer cattle and sheep and just eat more kangaroos.

　　The_idea is controversial（争议），but about 20 percent of health conscious Australians are believed to eat the national symbol already.

　　“It's low in fat，it's got high protein levels and it's very clean in the sense that basically it's the ultimate free range animal（散养动物），” said Peter Ampt of the University of New South Wales's institute of environmental studies.

　　1．According to the passage，global warming is now mainly caused by ______.

　　A．methane

　　B．carbon dioxide

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