Although most species of nondomestic

Answer choices

1. A

   Too Weak11% picked this

   Some marsupials now found in Australia might not be native to that continent, but rather might have been introduced to

   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%
2. B

   Too Weak9% picked this

   Humans who settled Australia probably introduced many of the placental mammal species now present

   "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%
3. C

   Correct67% picked this

   The only Australian placentals that could be native to Australia would be animals whose ancestors could not have

   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%
4. D

   Unrelated to Goal12% picked this

   No marsupials now found in Australia can swim long distances, fly, or

   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%
5. E

   Too Weak1% picked this

   Seals, bats, and mice are typically found only in areas where there are

   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%