Remember, we want an assumption that is going to guarantee our conclusion.
Conclusion: It is clear, however, that these placentals are not native to this island continent.
Why?
Support: Because all nonhuman placentals, except the dingo, have ancestors who could swim long distances, fly, or float on driftwood.
Let's discuss answer choice D.
What if none of the marsupials in this area could travel this way? Does that prove our conclusion? No, because it doesn't tell us anything about placentals. They are the subject of our conclusion.
C is correct because, if true, it confirms that the placentals in question are not native. A placental is native (N) only if its ancestors could not have traveled from afar (not TFA).
N - - - - - - - - - - - > not TFA
We know that their ancestors could have traveled from afar.
TFA - - - - - - - - -> not N
We can rightfully conclude that they are not native.
