Which of the following best describes why polar bears are a sub-species of brown bears instead of a completely separate species?
A. Because they both have thick fur and 4 legs, so they are very similar
B. Because they both live on the same continent, so they can’t be two separate species
C. Because they can still breed in the wild. Once they no longer mate with brown bears, they will be considered a separate species
D. All of the above