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It doesn't. The conclusion of this argument is, "It is clear, however, that these placentals are not native to this island continent." And how do we know this? We are told that all nonhuman placentals, except the dingo which was introduced by the first humans who settled in Australia, are animals whose ancestors could swim long distances, fly, or float on driftwood. We are looking for the answer choice that 100% guarantees the conclusion of the argument. Let's take a closer look at answer choice (C), which states, "the only Australian placentals that could have been native to Australia would be animals whose ancestors could not have reached Australia from elsewhere." We know that "the only" introduces sufficient so this answer choice would be diagrammed as follows: PNA ===> not RAE RAE ===> not PNA PNA = placentals native to Australia RAE = reached Australia from elsewhere We know from the stimulus that all nonhuman placentals, except the dingo, are animals whose ancestors could swim long distances, fly or float on driftwood. This would invoke the sufficient condition of the contrapositive of answer choice (C) and allow us to conclude with 100% certainty that these placentals are not native to Australia. As such, answer choice (C) 100% guarantees the conclusion of the argument so it would be the correct answer. Hope this helps! Please let us know if you have any other questions.

Thanks a lot!!!! Wish you have a wonderful Thanksgiving over there!!!!

I see here that the person who first commented on this question was pointing out that the answer (I am assuming per the textbook) says that the answer should be D.. since this person referenced "No Marsupials now found in Australia can swim long distances, fly, or float on driftwood." Just so you guys are aware the textbook still says that the answer to this question should be answer choice D but your explanation says it should be C... just thought someone might want to be aware so that this can fixed since this has apparently been an issue for 5 years now..

@Margaret thanks for letting us know. The correct answer is (C). We will confirm that the hardcopy materials are also correct. Sorry for the confusion.