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Farmers in the United States sometimes plant switch grass as a border crop. But could this tall grass lower the nation’s dependence on foreign oil?

The Department of Energy plans to invest hundreds of millions of dollars to help produce fuels from materials that are not part of the food supply. Growing corn, or maize, for fuel has raised concerns about the supply and cost of corn available for food and animal feed.

Fuel made from switch grass or forestry waste like sawdust（鋸屑） is known as cellulosic（有纖維質的） ethanol（酒精）. Department officials say it contains more energy and produces fewer greenhouse gases than ethanol made from corn. Switch grass is also easier to grow.

Last month, the Proceedings of the National Academy of Sciences published a study of switch grass grown on low-quality land. Government scientist Ken Vogel was the lead author. The study says the switch grass produced five times more energy than was needed to grow it. Also, it says switch grass, over its lifetime from crop to fuel, produces much less carbon compared to gasoline.

Fossil fuels like oil take carbon from the ground and release it as waste gas when the fuel is burned. Biofuels（生物燃料） like corn and cellulosic ethanol also produce greenhouse gases, through growing crops and making the fuel. The difference is that biofuels remove carbon from the atmosphere through the growth of the feedstock, the material for the fuel.

Science magazine just published two studies of biofuels and the heat-trapping gases that scientists link to climate change. One of the reports notes that most studies have found that substituting biofuels for gasoline will reduce greenhouse gases.

But it says the earlier studies failed to count the carbon released into the atmosphere as farmers worldwide react to higher prices. They are clearing forests and grasslands to make way for new cropland to replace the grain used for biofuels. Doing so can release much of the carbon stored in the plants and soil, and sacrifice future storage.

The study found that corn-based ethanol could increase greenhouse gases for years from land use change. And it found that biofuels from switch grass, if grown on American corn land, could also increase emissions, though by less.

The study team, led by Timothy Searchinger at Princeton University, says the result shows the value of using waste products for fuel. The other report says carbon savings depend on how biofuels are produced.

1．Switch grass is a better fuel than corn in all the following aspects except         .

       A．switch grass grows on low – quality land

       B．switch grass produces more energy than corn

       C．switch grass produces fewer greenhouse gases than corn

       D．switchgears is easier to grow

2．It is implied in the underlined sentence that         .

       A．an increasing number of people worry about the use of corn as fuel

       B．there will be an increase in price of corn for food if it is used as fuel

       C．there will be an decrease in quantity of corn for food if it is used as fuel

       D．all the above choices are right

3．Biofuels produces much less carbon compared to Fossil fuels in that         .

       A．biofuels produce less greenhouse gases than fossil fuels

       B．fossil fuels release carbon while biofuels not when burning

       C．biofuels take in carbon from the air when they are growing

       D．biofuels produce greenhouse gases only when they are burning

4．The passage indicates that           .

       A．we’d better make full use of biofuels to protect the environment

       B．the value of using biofuels remains to be further studied

       C．it’s costless to grow biofeuls

       D．biofuels are just switchgrass and corn