求问prep07 cr2中一道题

lexuefeifelix

22. (28162-!-item-!-188;#058&003858)

Herbicides allow cereal crops to be grown veryefficiently, with virtually no competition from weeds.  In Britain

, partridge populations havebeen steadily decreasing since herbicide use became widespread.  Some environmentalists claim that thesebirds, which live in and around cereal crop fields, are being poisoned by theherbicides.  However, tests show no morethan trace quantities of herbicides in partridges on herbicide-treatedland.  Therefore, something other thanherbicide use must be responsible for the population decrease.

Which of the following, if true about Britain

,most seriously weakens the argument?

(A) The elimination of certain weeds from cerealcrop fields has reduced the population of the small insects that live on thoseweeds and that form a major part of partridge chicks' diet.

(B) Since partridges are valued as game birds,records of their population are more carefully kept than those for many otherbirds.

(C) Some of the weeds that are eliminated fromcereal crop fields by herbicides are much smaller than the crop plantsthemselves and would have no negative effect on crop yield if they were allowedto grow.

(D) Birds other than partridges that live in oraround cereal crop fields have also been suffering population declines.

(E) The toxins contained in herbicides typicallyused on cereal crops can be readily identified in the tissues of animals thathave ingested them.

我猜的是A，答案也是A，因为只有A说的通，可我有个问题..

结论是，herbicide is not responsible for the decline,不过原文也说了，专家认为是herbside毒死了它们造成decline，结论说，肯定有其他原因..那这个因为herbcide杀死了杂草进而减少了它们的食物来源不就正好是一个other explanation??也可以support啊...

jyz2390

weaken 题两种常用方法：它因法和断桥法即是找到前提和结论间的差异，使前提不能推出结论。
结论是：是某种除了除草剂以外的东西，导致了birds decline. 我们要weaken 它，就是要证明，就是除草剂导致了生物的减少。
A 正确，因为除草剂除掉了以这种草为生的虫子，同时虫子也是那些birds 赖以为生的食物，当然birds数量就会减少了。
正是weaken 了结论说不是除草剂使birds数量减少。