The Naturalist and Father of Zoogeography Alfred Russel Wallace Noted

The naturalist and father of zoogeography Alfred Russel Wallace noted a distinct difference in the fauna between several islands in the Malay Archipelego (present-day Indonesia) . He used these differences to delineate the boundary between the Australian and Oriental biogeographic realms. Which of the following best describes the reason for these differences?

A) Aboriginal tribes in Australia have greatly altered the fauna of Australia relative to the Oriental realm.
B) A deep ocean barrier inhibited dispersal between the two realms.
C) The environments of Australia and Asia are completely different, despite their relative close proximity.
D) Asia and European traders interacted for centuries prior to Wallace's discovery. Among the results of this interaction was an exchange of European (Palearctic) fauna and Oriental fauna.