有一道Weaken需要各位的解释，请帮忙！THX

Herbicides allow cereal crops to be grown very efficiently, with virtually no competition from weeds.  In Britain, partridge populations have been steadily decreasing since herbicide use became widespread.  Some environmentalists claim that these birds, which live in and around cereal crop fields, are being poisoned by the herbicides.  However, tests show no more than trace quantities of herbicides in partridges on herbicide-treated land.  Therefore, something other than herbicide 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 cereal crop fields has reduced the population of the small insects that live on those weeds 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 other birds.

(C) Some of the weeds that are eliminated from cereal crop fields by herbicides are much smaller than the crop plants themselves and would have no negative effect on crop yield if they were allowed to grow.

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

(E) The toxins contained in herbicides typically used on cereal crops can be readily identified in the tissues of animals that have ingested them.

这题我选了D,因为我觉得要削弱就要表明除了herbicide之外就没有其他原因导致partridge的数量下降，即使是对于partridge以外的动物而言也是这样。

但正确的答案是选A,我怎么看都觉得A是削弱environmentalists‘ claim，不像是削弱本文的论述。有谁可以帮帮忙呢？十分多谢您的帮助！！！！！

原文的逻辑是:

没有在partridge体内没trace到herbicide -> herbicide跟partridge的decrease无关

D选项是无关项。D说其它鸟的数量也减少了，跟结论没有任何关系。

你之所以会觉得D选项是削弱项是因为你自己潜意识里做了 “herbicide  -> 其他鸟的数量减少”这么一个assumption，这是不对的，D选项里没有明确指明的逻辑千万不要加进去。例如 这里如果我读D选项时暗地里做一个"他因 -> 其它鸟数量减少"的assumption的话岂不是D选项也可以是加强选项了？

A里面暗藏了一个逻辑，但这个逻辑是一个事实是一个公理（也就是说在地球范围内长期认为是对的），即use herbicide ->elimination of weeds （使用除草剂 -> 将会除掉草）

A想要表达的逻辑是：use herbicide -> elimination of weeds -> reduce a small insect -> main food of partridge reduced -> partridge reduced

A实际上就是削弱选项，再看一遍原文的逻辑：partridge体内没trace到herbicide -> herbicide跟partridge的decrease无关。 这里A选项削弱了原文的evidence，即使“partridge体内没trace到herbicide”也不能说herbicide就跟partridge的decrease无关，A就是找到了另一个途径来说明herbicide跟partridge的decrease有关,是通过虫子来牵线搭桥的间接有关