For as long as humans have raised crops as a source of food and other products, insects have damaged them. Between 1870 and 1880, locusts ate millions of dollars' worth of crops in the Mississippi Valley. Today in the United States the cotton boll weevil damages about 300 million dollars' worth of crops each year. Additional millions are lost each year to the appetites of other plant-eating insects. Some of these are corn borers, gypsy moths, potato beetles, and Japanese beetles.

　　In modern times, many powerful insecticides(殺蟲劑) have been used in an attempt to destroy insects that damage crops and trees. Some kinds of insecticides, when carefully used, have worked well. Yet the same insecticides have caused some unexpected problems. In one large area, an insecticide was used against Japanese beetles, which eat almost any kind of flower or leaf. Shortly afterward, the number of corn borers almost doubled. As intended, the insecticide had killed many Japanese beetles. But it had killed many of the insect enemies of the corn borer as well.

　　In another case, an insecticide was used in Louisiana to kill the troublesome fire ant. The insecticide did not kill many fire ants. It did kill several small animals. It also killed some insect enemies of the sugarcane borer, a much more destructive pest than the fire ants. As a result, the number of sugarcane borers increased and severely damaged the sugarcane crop.

　　To be sure that one insect pest will not be traded for another when an insecticide is used, scientists must perform careful experiments and do wide research. The experiments and research provide knowledge of the possible hazards an insecticide may bring to plant and animal communities. Without such knowledge, we have found that nature sometimes responds to insecticides in unexpected ways.

　　21. An insecticide was used in Louisiana to kill the troublesome______.

　　A. corn borer B. Japanese beetle

　　B. gypsy moth D. fire ant

　　22. While it is not directly stated, the article suggests that______.

　　A. insecticides are not dangerous to any small animals

　　B. insecticides do not always accomplish their purposes

　　C. insecticides are no longer being used to kill insects

　　D. insecticides do no harm to people

　　23. On the whole, the article tells about______.

　　A. the appetites of plant-eating insects B. the best way to kill boll weevils C. the dangers in using insecticides D. the best way to grow crops

　　24. Which statement does this article lead you to believe?

　　A. All changes are predictable. B. Nothing ever changes in nature.

　　C. Nature is not always predictable. D. Nature always serves man well.

　　25. Scientists perform careful experiments and do wide research because______.

　　A. they must learn to destroy all the insects that we need

　　B. they must be sure one insect pest is not traded for another

　　C. research keeps them from inventing new insecticide for the crops

　　D. research helps them find a way to kill all insects

　　答案：21. D 22. B 23. C 24. C 25. B