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  3. The world seems increasingly divided into those who favor genetically modified (GM) foods and those who fear them. Advocates ass

The world seems increasingly divided into those who favor genetically modified (GM) foods and those who fear them. Advocates ass

admin2012-04-23  10
问题     The world seems increasingly divided into those who favor genetically modified (GM) foods and those who fear them. Advocates assert that growing genetically altered crops can be kinder to the environment and that eating foods from those plants is perfectly safe. And, they say, genetic engineering—which can induce plants to grow in poor soils or to produce more nutritious foods—will soon become an essential tool for helping to feed the world’s burgeoning population. Skeptics contend that GM crops could pose unique risks to the environment and to health—risks too troubling to accept placidly. Taking that view, many European countries are restricting the planting and importation of GM agricultural products. Much of the debate hinges on perceptions of safety. But what exactly does recent scientific research say about the hazards?
    Advocates of GM, or transgenic, crops say the plants will benefit the environment by requiring fewer toxic pesticides than conventional crops. But critics fear the potential risks and wonder how big the benefits really are. "We have so many questions about these plants," remarks Guenther Stotzky, a soil microbiologist at New York University. "There’s a lot we don’t know and need to find out. "
    As GM crops multiply in the landscape, unprecedented numbers of researchers have started fanning into the fields to get the missing information. Some of their recent findings are reassuring; others suggest a need for vigilance.
    Every year U.S. growers shower crops with an estimated 971 million pounds of pesticides, mostly to kill insects, weeds and fungi. But pesticide residues linger on crops and the surrounding soil, leaching into groundwater, running into streams and getting gobbled up by wildlife. The constant chemical trickle is an old worry for environmentalists.
    In the mid-1990s agribusinesses began advertising GM seeds that promised to reduce a farmer’s use of toxic pesticides. Today most GM crops—mainly soybean, corn, cotton and canola—contain genes enabling them to either resist insect pests or tolerate weed-killing herbicides. The insect-resistant varieties make their own insecticide, a property meant to reduce the need for chemical sprays. The herbicidetolerant types survive when exposed to broad-spectrum weed killers, potentially allowing farmers to forgo more poisonous chemicals that target specific weed species. Farmers like to limit the use of more hazardous pesticides when they can, but GM crops also hold appeal because they simplify operations (reducing the frequency and complexity of pesticide applications) and, in some cases, increase yields.
    But confirming environmental benefit is tricky. Virtually no peer-reviewed papers have addressed such advantages, which would be expected to vary from plant to plant and place to place. Some information is available, however. According to the U.S. Department of Agriculture, farmers who plant herbicidetolerant crops do not necessarily use fewer sprays, but they do apply a more benign mix of chemicals. For instance, those who grow herbicide-tolerant soybeans typically avoid the most noxious weed killer, turning instead to glyphosate herbicides, which are less toxic and degrade more quickly.
    Insect-resistant crops also bring mixed benefits. To date, insect resistance has been provided by a gene from the soil bacterium Bacillus thuringiensis (Bt). This gene directs cells to manufacture a crystalline protein that is toxic to certain insects—especially caterpillars and beetles that gnaw on crops—but does not harm other organisms. The toxin gene in different strains of Bacillus. thuringiensis can affect different mixes of insects, so seed makers can select the version that seems best suited to a particular crop.
    Defining the environmental risks of GM crops seems even harder than calculating their benefits. At the moment, public attention is most trained on Bt crops, thanks to several negative studies. Regulators, too, are surveying the risks intensely. This spring or summer the EPA is expected to issue major new guidelines for Bt crops, ordering seed producers to show more thoroughly that the crops can be planted safely and monitored in farm fields.
    In the face of mounting consumer concern, scientists are stepping up research into the consequences of Bt and other GM crops. Among their questions: How do Bt crops affect "nontarget" organisms—the innocent bugs, birds, worms and other creatures that happen to pass by the modified plants? Will GM crops pollinate nearby plants, casting their genes into the wild to create superweeds that grow unchecked? What are the odds that the genetically engineered traits will lose their ability to protect against insects and invasive weeds, leaving GM plants suddenly vulnerable?  
What is the environmental benefit of GM crops?
选项 A、Herbicidetolerant crops can reduce the use of chemicals,
B、Herbicidetolerant crops can use milder chemicals.
C、Bt genes can protect crops against insects instead of chemical pesticides.
D、Insect-resistant crops only need a more benign mix of chemicals.
答案B
解析 第六段中明确指出耐除草剂的作物不一定降低农药用量，但可以使农民降低农药毒性。故答案为B。而下文中提到不同Bt基因各有不同的毒基因，影响的昆虫也不同，所以只能针对特定的昆虫，不一定能完全替代杀虫剂，选项C是在原文基础上的延伸，不确切。D选项将耐除草剂的作物与抗虫害作物的功能混淆了。
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