53. In Australia, marsupials fill the niches that placental mammals fill in other parts of the world because ________. (A) after Pangaea broke up, they diversified in physical isolation from placental mammals (B) they originated in Australia (C) they evolved from monotremes that migrated to Australia about 50 million years ago (D) human-caused environmental changes have favored the success of marsupials (E) they are better adapted and have outcompeted placental mammals (eutherians)