With petroleum reserves (石油储量) decreasing，the search is on to replace gas with a cleaner，greener alternative.Though much eco－talk has centered on biofuels from corn and soybeans，the biofuel that looks more likely to replace petroleum comes from a most unlikely source：algae (藻类)． Algae，like corn，soybeans，and other crops，grow via photosynthesis (光合作用) and can be processed into fuel oil.However，they yield 30 times more energy than land crops such as soybeans，according to the U．S.Department of Energy.Many algae species also can grow in saltwater and polluted water－while corn and soy require arable land and fresh water that will be in short supply as the world's population balloons. “If you replaced all the diesel (柴油) in the U．S.with soy biodiesel，it would take half the land mass of the U．S.to grow those soybeans，”says Matt Caspari，chief executive of Aurora Biofuels，a Berkeley，Calif.－based private firm that specializes in algae oil technology.On the other hand，the Energy Department says that if algae fuel replaced all the petroleum fuel in the United States，it would require 15,000 square miles，which is a few thousand miles larger that Maryland (12,407 square miles)，the 42nd state in land area. Another bonus：Because algae can be grown just about anywhere in a closed space，they 're being tested at several power plants across the nation as a carbon absorber.Smokestack emissions (烟囱排放物) can be pumped directly into the ponds，feeding the algae while keeping greenhouse gases out of the atmosphere. Although processing technology for algae fuel—also known as “oilgae” in some environmentalist circles—is improving，it's still years away from reaching your local gas pump.“It's just a question of cost，because no large－scale facilities have been built yet，” Caspari says. 小题1:The underlined word “yield” in Paragraph 2 can be replaced by “________”．
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