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 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
21. D 22. B 23. C 24. C 25. B
从心理角度提高四六级听力成绩
通过英语四级听力的两大妙招
四六级考前30天阅读冲刺要重真题找技巧
英语四级写作难点分析及应对策略
英语四六级快速阅读高分突破四步走
英语四六级考前三十天听力冲刺方法全指导
辨析六级考试完形填空逻辑衔接词
英语四六级词汇积累和熟练是基础
12月英语六级冲刺考前一周复习要点提示
英语四六级考试指导穿越四六级翻译
如何炼成英语四六级应试指导作文达人
四六级听力不给力的原因和解决办法
名师分析英语四级翻译主要考点
徐可风6月英语六级试题难度持平
点评英语四级真题
赵建昆指导考前五天该如何备考英语四六级
英语四六级考前三十天常遇困惑全解密
名师指导英语四级考试全题型备考
英语四级考试各题型特点及冲刺指导
英语四级写作指导三段结构
6月英语四级考试如何高速浏览文章
英语六级考试阅读备考常见的问题
6月英语四级考试阅读七大技巧
6月英语四六级考前温馨提示
写好英语句子的方法
12月四六级作文考前一周提分真经
6月英语四级考试完型填空题做题技巧
名师支招12月英语四级最后冲刺全攻略
英语六级考试阅读真题的英语典故探析
英语四六级考前作文辅导
不限 |
英语教案 |
英语课件 |
英语试题 |
不限 |
不限 |
上册 |
下册 |
不限 |