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.
申友解析：P + CP → C
Herbicides → weeds ↓
↘ partridge ↓ other cause → partridge ↓
Trace quantities of herbicides in partridge
Weaken 结论，即削弱 CP，增强 P，即Partridge的减少就是由于Herbicide的原因，所以找weeds与partridge之间的关系增强
(B) game bird 原文无关，排除
(C) weeds 和 crop 比较，跳出原文讨论范畴，排除
(E) animal 扩大讨论范围，排除