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______.

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