2. 外语
  3. Every year U. S. growers shower crops with an estimated 971 million pounds of pesticides, mostly to kill insects and weeds. But

Every year U. S. growers shower crops with an estimated 971 million pounds of pesticides, mostly to kill insects and weeds. But

admin2013-07-02  15
问题     Every year U. S. growers shower crops with an estimated 971 million pounds of pesticides, mostly to kill insects and weeds. 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(农业综合企业)began advertising GM seeds that promised to reduce a farmer’s use of toxic pesticides. Today most GM crops — mainly soybean, corn and cotton — contain genes enabling them to either resist insect pests or tolerate weed-killing herbicides(除草剂). The insect-resistant varieties make their own insecticide, a property meant to reduce the need for chemical sprays. The herbicidetoler-ant 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.
    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 but does not harm other organisms. The toxin gene in different strains of Bt 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?
From the passage, we can learn that plants which can tolerate herbicide______.
选项 A、need no chemical pesticides in the whole process of maturing
B、need less chemical pesticides as they were advertised before
C、can turn out some certain chemical pesticides by themselves
D、require gentle chemicals and simple planting as well
答案D
解析 事实细节题。第三段指出基改植物“对环境有好处”却不易证实。但还是有资料表明：耐除草剂的作物不见得会降低农药的喷洒量，不过农民确实在使。用比较温和的混合药剂，第二段提到基改种子简化了劳作程序，使种植简单了。所以选项[D]正确。其他选项都与文章所提的事实不符。
转载请注明原文地址:https://www.kaotiyun.com/show/uSn7777K
0

大学英语六级

相关试题推荐
随机试题
最新回复(0)