Official Explanation
Fact #1 – Arctic swan number rapidly declining
Fact #2 – Dying from lead poisoning from buckshot
Fact #3 – Authorities have banned hunting
Paradox: birds are continuing to decline at dramatic rate
(A) is focused on various toxins. The argument clearly says “the number of birds…die each month from lead poisoning”.
(B) only deepens the mystery since the hunters still entering the park aren’t killing more swan than previous hunters. If they were, then that could account for the fact that the number of swans continues to decrease.
(C) doesn’t quite resolve the paradox. It hints at the fact that there might be more hunters than previously thought still entering the park. But just how many? Is that number significant enough to account for the ongoing decrease in the swan’s numbers? Too many questions and no clear answer.
(D) resolves the paradox. The swan is not dying because it is being shot. It is dying because it is eating buckshot that has been lying on the ground all along. Unless, somebody cleans up all the buckshot pellet, the swan will continue to die, even if no more hunters enter the park.
(E) just tells us that the swan’s predicament is dire. It does provide a reason that the swan’s numbers continue to decline despite the ban on hunting.