Herbicides allow cereal crops to be grown very efficiently, with virtually no competition for weeds. In Britain, the 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 and 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

conclusion:something OTHER THAN herbicide use must be responsible for the population decrease.

we have to weaken this means we have to show that population decrease is because of the herbicide use (and not other than herbicide use) .

option A:The elimination of certain weeds from cereal crop fields has reduced the population of the SMALL INSECTS that live on those weeds and thatform a major part of partridge chicks’ diet.==>So here insects are dying because of herbicides..and insects are major part of birds diet.==>means birds are lacking in their food and because of this they are dying...===>so overall it is effect of HERBICIDES ONLY..hence it weakens==>correct

option E:The toxins contained in herbicides typically used on cereal crops can be readily identified in the tissues of animals that have ingested them ==>MEANS if somebody ingested that toxins it can be readily find in the tissues===>its stating fact====>that if you eat 5gms of toxins then it can be detected on your tissues...does this say it is poisoning birds...or what amount does this will poison bird.....so its stating a fact====and hence not affecting the conclusion.