Logical Reasoning

PT18 · S4 · Q12 Although most species of nondomestic

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Although most species of nondomestic mammals in Australia are marsupials, over 100 species—including seals, bats, and mice—are not marsupials but placentals.

Conclusion

Placentals are not native to Australia.

Evidence

All placentals (except the dingo, which came with humans) had ancestors who could have swum to Australia, flown there, or floated there on driftwood.

Evaluate

Just because these placentals could have gotten to the island of Australia by flying, swimming, or floating doesn't show that this is how it happened.

It's possible that these placentals are native to Australia.

Goal

We need some answer that would eliminate the possibility that the placentals started out in Australia.

It's hard to predict what the correct answer will sound like, other than something like .

12.

The conclusion above is properly drawn if which one of the following is assumed?

  1. Some marsupials now found in

    Too Weak

    I honestly saw the words "some" and "might" at a bird's eye level and dismissed this answer out of hand. We need an incredibly powerful answer choice that guarantees that no placentals are native to Australia. This answer is incredibly weak in strength (at least one X might not be Y) and isn't even about placentals.

    11% picked this

  2. Humans who settled Australia probably

    Too Weak

    "Many" is again too weak. We need an answer that guarantees that none of the placentals are native to Australia. This answer would allow us to say that "many of the placentals are probably not native", but that's nowhere near proving the conclusion.

    9% picked this

  3. Correct

    The only Australian placentals that

    Why this is right

    "The only" is a sufficient indicator. The idea attached to it goes on the Left of the arrow. Australian placental ? ancestors could not native to Australia have reached AU from elsewhere Contrapositive? if ancestors could then not native have reached AU ? to Australia from elsewhere Do we know that these Australian placentals could have reached Australia from elsewhere? Sure, the premise says as much (all nonhuman placentals are animals whose ancestors could swim long distances, fly, or float). So according to the rule this answer choice provides, these Australian placentals could not be native to Australia. (To be fair, it's not a perfect match to say that just because the ancestors could swim a long distance, fly, or float on driftwood, that they could reach Australia from elsewhere. I can float on driftwood too, but I won't survive the weeks-long trip from the Indian subcontinent, or even from New Zealand)

    Skill tested: Sufficient Assumption · how this choice captures the argument's function is the move to repeat next time.

    67% picked this

  4. No marsupials now found in

    Unrelated to Goal

    We're trying to prove that no placentals are native to Australia. Any fact we learn about marsupials is of zero usefulness to us.

    12% picked this

  5. Seals, bats, and mice are

    Too Weak

    There are a handful of problems with this answer, but the strength of typically is enough to kill it quickly. "typically" just means more than 50% of the time, but we need to prove that 100% of the placental mammals in Australia did not originate there. We should also learn to be suspicious of answers that act like off-the-cuff examples (100 placentals, including seals / bats / mice) are somehow crucial to the author's logic. Even if we changed "typically" to "always", this answer would still just be about those three species, but our job is to prove all 100 aren't native to Australia. Finally, the marsupials in Australia are identified as non-native (nondomestic), so this answer doesn't even seem to have the effect we'd want. If we knew that Australia had native marsupials (we don't -- the paragraph never said they do), then this would suggest that seals/bats/mice would probably not be found there. Moreover, "whether or not your found somewhere" is very different from "whether or not you originated there".

    1% picked this

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