托福阅读official38 Look at the four squares [[■]] that indicate where the following sentence could be added to the pa答案解析【小站备考】

Genes from virtually any organism, from viruses to humans, can now be inserted into plants, creating what are known as transgenic plants. Now used in agriculture, there are approximately 109 million acres of transgenic crops grown worldwide, 68 percent of which are in the United States. The most common transgenic crops are soybeans, corn, cotton, and canola. Most often, these plants either contain a gene making them resistant to the herbicide glyphosate or they contain an insect-resistant gene that produces a protein called Bt toxin.

On the positive side, proponents of transgenic crops argue that these crops are environmentally friendly because they allow farmers to use fewer and less noxious chemicals for crop production. For example, a 21 percent reduction in the use of insecticide has been reported on Bt cotton (transgenic cotton that produces Bt toxin). In addition, when glyphosate is used to control weeds, other more persistent herbicides do not need to be applied.

On the negative side, opponents of transgenic crops suggest that there are many questions that need to be answered before transgenic crops are grown on a large scale. One question deals with the effects that Bt plants have on nontarget organisms such as beneficial insects, worms, and birds that consume the genetically engineered crop. For example, monarch caterpillars feeding on milkweed plants near Bt cornfields will eat some corn pollen that has fallen on the milkweed leaves. Laboratory studies indicate that caterpillars can die from eating Bt pollen. However, field tests indicate that Bt corn is not likely to harm monarchs. Furthermore, the application of pesticides (the alternative to growing Bt plants) has been demonstrated to cause widespread harm to nontarget insects.

Another unanswered question is whether herbicide-resistant genes will move into the populations of weeds. Crop plants are sometimes grown in areas where weedy relatives also live. If the crop plants hybridize and reproduce with weedy relatives, then this herbicide-resistant gene will be perpetuated in the offspring. [■] In this way, the resistant gene can make its way into the weed population. [■] If this happens, a farmer can no longer use glyphosate, for example, to kill those weeds. [■] This scenario is not likely to occur in many instances because there are no weedy relatives growing near the crop plant. [■] However, in some cases, it may become a serious problem. For example, canola readily hybridizes with mustard weed species and could transfer its herbicide-resistant genes to those weeds.

We know that evolution will occur when transgenic plants grown on a large scale over a period of time. Of special concern is the development of insect populations resistant to the Bt toxin. This pesticide has been applied to plants for decades without the development of insect-resistant populations. However, transgenic Bt plants express the toxin in all tissues throughout growing season. Therefore, all insects carrying genes that make them susceptible to the toxin will die. That leaves only the genetically resistant insects alive to perpetuate the population. When these resistant insects mate, they will produce a high proportion of offspring capable of surviving in the presence of the Bt toxin. Farmers are attempting to slow the development of insect resistance in Bt crops by, for example, planting nontransgenic border rows to provide a refuge for susceptible insects. These insects may allow Bt susceptibility to remain in the population.

Perhaps the most serious concern about the transgenic crop plants currently in use is that they encourage farmers to move farther away from sustainable agricultural farming practices, meaning ones that allow natural resources to continually regenerate over the long run. Transgenics, at least superficially, simplify farming by reducing the choices made by the manager. Planting a glyphosate-resistant crop commits a farmer to using that herbicide for the reason, probably to the exclusion of all other herbicides and other weed-control practices. Farmers who use Bt transgenics may not feel that they need to follow through with integrated pest-management practices that use beneficial insects and timely applications of pesticides to control insect pests. A more sustainable approach would be to plant nontransgenic corn, monitor the fields throughout the growing season, and then apply a pesticide only if and when needed.

OFFICIAL38 Look at the four squares [[■]] that indicate where the following sentence could be added to the passage. Where would the sentence best fit? Click on a square [[■]] to insert the sentence in the passage.

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