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  3. In an intact plant community, undisturbed by human intervention, the composition of a community is mainly a function of the clim

In an intact plant community, undisturbed by human intervention, the composition of a community is mainly a function of the clim

admin2011-02-11  66
问题   In an intact plant community, undisturbed by human intervention, the composition of a community is mainly a function of the climate and the type of soil. Today, such original communities are very rare—they are practically limited to national parks and reservations.
  Civilization has progressively transformed the conditions determining the composition of plant communities. For several thousand years vast areas of arable land have been hoed, ploughed, harrowed and grassland has been cut or grazed. During the last decades the use of chemical substances, such as fertilizers and most recently of weed killers (herbicides) has greatly influenced the composition of weed communities in farm land.
  All selective herbicides have specific ranges of activity. They control the most important weeds but not all the plants of a community. The latter profit from the new free space and from the fertilizer as much as the crop does; hence they often spread rapidly and become weeds unless another herbicide for their eradication is found.
  The soil contains enormous quantities of seeds of numerous specie—up to half a million per square meters according to scientific literature—that retain their ability to germinate for decades. Thus it may occur that weeds that were hardly noticed before emerge in masses after the elimination of their competitors. Hence, the knowledge of the composition of weed communities before selective weed killers are applied is not only of scientific interest since the plant species present in the soil in the form of seeds must be considered as potential weeds. For efficient control the identification of weeds at the seedling stage, i.e. at a time when they can still be controlled, is particularly necessary; for the choice of the appropriate herbicide depends on the composition of the weed community.
Why are there problem weeds?
选项 A、Because they cannot be eradicated by herbicides.
B、Because all selective herbicides can encourage the growth of previously unimportant weeds by eliminating their competitors.
C、Because they were hardly considered before so that their seeds were not prevented from germinating.
D、Because they benefit greatly from the fertilizer applied to the farm land.
答案D
解析 本题为细节判断题。原文的第三段告诉我们：所有选用的除草剂都只适用于一定的范围，它们往往能控制住最需要控制的野草，但并非所有。那些没被控制住的草便从庄稼和化肥中获得了新的生长空间。如果没有找到根除它们的除草剂的话，它们就会迅速蔓延开去。选项D与原文相符，为最佳答案。
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