题目选项分析

The number of Tundra swan in an arctic refuge has been steadily dwindling. It has been determined that the majority of the birds have died from lead poisoning, which is contained in the buckshot that hunters use to kill the swan. Local authorities have responded by banning hunting, and the number of hunters cited has since dropped dramatically, yet the number of birds that continue to die each month from lead poisoning has not changed.

Which of the following, if true, best resolves the discrepancy above?

【选项】Migrating tundra swans fly through Idaho where they pick up various toxins from the silver mines there.