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Aviation fuel emerged as the largest source of lead emissions in the U. S. once lead was phased out of automotive gasoline begin

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问题     Aviation fuel emerged as the largest source of lead emissions in the U. S. once lead was phased out of automotive gasoline beginning in the 1970s.【R1】______
    Some of the health effects of repeated exposure to lead include damage to the central nervous system, kidneys and red blood cells, and decreased function in the cardiovascular and immune systems.【R2】______And scientists at the National Toxicology Program have concluded that lead and lead compounds are "reasonably anticipated to be human carcinogens. "
    The U.S. Environmental Protection Agency(EPA)recognizes lead as a neurotoxin and in 2008 set tough new standards for how much of it is safe in our air.【R3】______But the EPA has not yet restricted lead in avgas, even though unleaded avgas is available.
    A 2011 Duke University study found that kids living within 500 meters of an airport where leaded avgas is used have higher blood lead levels than other children, with elevated lead levels in blood found in kids as far as one kilometer away. The EPA estimates that 16 million Americans live close to one of 22,000 airports where leaded avgas is routinely used—and three million children go to schools near these airports.
    【R4】______That original petition requested that the EPA issue a finding that emissions from aircraft using leaded avgas endanger public health. "EPA has repeatedly concluded that lead is extremely toxic to humans, wildlife and the environment and causes health effects even at low doses," says Marcie Keever, FoE’s legal director. "EPA’s continuing failure to do what the law requires and address this pollution leaves us no choice but to take this critical public health issue to the courts. "
    According to FoE, 70 percent of small planes could already be using unleaded avgas with no retrofitting needed. The group says that a meaningful plan by the EPA to ban leaded avgas could spark investment in technologies to replace the engines in the rest of the small plane market that relies on leaded avgas.
    Some members of the aviation community are taking matters into their own hands.【R5】______Green groups are pleased with this development, but want the U. S. government to institute binding restrictions on the use of lead in aviation fuel.
[A]The Aviation Fuel Club, which aims to make aviation fuel affordable for sport aviators, is working to ensure that unleaded avgas is available at many airports across the country.
[B]In 2010, the agency still identified 16 U. S. regions that fail to meet clean air standards for airborne lead; all either contained or were near airports where leaded avgas is the norm.
[C]Lead was long ago phased out of automobile gasoline, but it is still in aviation fuel and is now the largest source of lead emissions in the U.S.
[D]Friends of the Earth(FoE), a leading green group, filed suit against the EPA in late 2011, demanding that it respond to a petition originally submitted in 2006 asking for regulation of lead emissions from general aviation aircraft under the Clean Air Act.
[E]Aviation fuel is generally of a higher quality than fuels used in less critical applications, such as heating or road transport, and often contains additives to reduce the risk of icing or explosion due to high temperatures, among other properties.
[F]While jets, which comprise the majority of commercial aircraft, don’t use leaded fuel, smaller, piston-engine planes use enough leaded aviation fuel(nicknamed "avgas")to account for half of the lead pollution in American skies, making it a real air quality issue.
[G]Lower IQ levels and learning disabilities can also result from lead exposure, especially in children, whose young bodies are more sensitive than those of adults.
【R2】
选项
答案G
解析 本题空格前后讲述的内容均是关于过度地接触铅会对人体产生的伤害，例如首句指出铅对中枢神经系统、肾脏、心血管等功能的破坏，末句提到科学家们甚至认为铅和铅化合物是“人类致癌的凶手”。本题的选项应与上下文连贯，主要意思应该也是关于铅对人体的健康会产生的危害。[G]的大意是铅污染会降低人类的IQ，或是引发学习障碍，该项中还特别指出儿童的敏感体质使得他们更容易受到铅的危害。与上下文的信息完全吻合，所以[G]为正确答案。
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