Why Are Australian Mammals All Marsupial While Mammals on Other

Why are Australian mammals all marsupial while mammals on other continents are mostly placental?

A) The marsupial anatomy is better adapted to the environments of Australia than is the anatomy of placental mammals.
B) The placental anatomy is better adapted to the environments of North and South America than is the anatomy of marsupial mammals.
C) When the continents separated from one another, marsupials in Australia did not have the placental competitors that were present in the Americas.
D) Marsupials outcompeted the placental mammals in Australia.
E) Australian mammals have a different common ancestor than mammals elsewhere in the world.