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. Ttherefore, 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? ['Birds other than partridges that live in or around cereal crop fields have also been suffering population declines.', '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.', "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.", 'The toxins contained in herbicides typically used on cereal crops can be readily identified in the tissues of animals that have ingested them.']