10. When two populations of the same species which have been widely separated by a barrier meet, they may have changed enough to find it difficult to interbreed. They are thus on the verge of becoming separate species. Which of the following is not something that prevents interbreeding?
A) They look different.
B) Their offspring look like a mix of the two populations, but are still fertile.
C) They have different mating behaviors or languages.
D) Their sex cells are incompatible.
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