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?
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AMigrating tundra swans fly through Idaho where they pick up various toxins from the silver mines there.
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BThose hunters who continue to defy the hunting ban do not, on average, shoot more birds than did those who hunted in the park before the ban.
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CPark rangers have been unable to cite all of the the hunters that enter the park despite the ban.
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DMost swans that have perished in the refuge have died as a result of ingesting pellets scattered about the ground, which introduce lead into their bloodstreams.
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EEventually, there will be so few swan left in the refuge that the overall number of swan dying each month will decrease.
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正确答案: D