Until Approximately 200 Years Ago, Australians Lived in Virtual Isolation

Until approximately 200 years ago, Australians lived in virtual isolation from the rest of the world's population. Which of the following best explains this relative isolation?

A) Word spread among Indonesian mariners that Australia was a largely inhospitable landscape.
B) Australian populations were fierce warriors who successfully defended their lands from any and all invaders.
C) Rising sea levels during a period of global warming separated Australia from surrounding land masses.
D) During a period of global cooling, land bridges connecting Australia to the surrounding islands became submerged under impassible glaciers.