Although most species of nondomestic mammals in Australia are marsupials, over 100 species -- including seals, bats, and mice -- are not marsupials but placentals. It is clear, however, that these placentals are not native to this island continent: all nonhuman placentals except the dingo, a dog introduced by the first humans that settled Australia, are animals whose ancestors could swim long distances, fly, or float on driftwood. The conclusion above is properly drawn if which one of the following is assumed? ['The only Australian placentals that could be native to Australia would be animals whose ancestors could not have reached Australia from elsewhere.', 'Some marsupials now found in Australia might not be native to that continent, but rather might have been introduced to Australia by some other means.', 'Seals, bats, and mice are typically found only in areas where there are no native marsupials.', 'No marsupials now found in Australia can swim long distances, fly, or float on driftwood.']