Many people think the search for cleaner energy leads only to renewable resources like sun, wind and water．But it also leads to a fossil fuel．Natural gas is considered the cleanest of the fossil fuels, the fuel created by plant and animal remains over millions of years．Burning it releases fewer pollutants than oil or coal．The gas is mainly methane （甲烷）．It produces half the carbon dioxide of other fossil fuels．So it may help cut the production of carbon gases linked to climate change．
Russia is first in what are called "proved reserves" of natural gas．The United States is sixth．Over the years, big oil and gas companies recovered much of the easily reached supplies of gas in America．They drilled straight down into formations where gas collects．As these supplies were used up, big drillers looked for similar formations in other countries．
But now the industry is taking a new look．Companies are developing gas supplies trapped in shale （页岩） rock two to three thousand meters underground．They drill down to the shale, then go sideways and inject high-pressure water, sand or other material into the rock．This causes the rock to fracture, releasing the gas．Huge fields of gas shale are believed to lie under the Appalachian Mountains, Michigan and the south-central states．Gas shale exploration is being done mainly by small to medium sized companies．Eric Potter, a program director, says more than half the gas in the United States is now coming from these new reserves．
But hydraulic （液压的） fracturing can also produce debate and anger over the risk of groundwater pollution．This method of drilling is not federally supervised under the Safe Drinking Water Act．Some in Congress want to end that exemption （豁免） from the law．
Natural gas provides Americans with about one fourth of their energy．And, unlike oil, most of it is produced in America．Gas producers invested heavily in reaching new supplies when prices were high．But prices are down sharply now because the depression cut demand for energy．So energy expert Eric Potter says it is too early to know how the changing market prices will affect the market for gas shale exploration．
45．We can learn from the first paragraph that ________．
A．natural gas releases methane and carbon dioxide
B．natural gas is considered as the cleanest energy
C．natural gas is more environmentally friendly than other fossil fuels
D．natural gas is a renewable source of energy
46．The word “fracture” in Paragraph 4 probably means _______．
A．create         B．break          C．change         D．decrease
47．Gas shale exploration may cause disagreement because ______．
A．it may cause water pollution      B．it brings too high profits
C．it breaks the law                D．it is out of states’ control
48．According to Eric Potter, the new gas shale exploration ______．
A．will provide America with about one fourth of their energy
B．will increase demand for energy
C．will make gas producers invest a big sum of money
D．may be influenced by the changing market prices