If Two Populations Are Re-Introduced After Being Genetically Isolated for a Long

If two populations are re-introduced after being genetically isolated for a long time, how could you tell if they've become two different species?

A) If they fight with each other, they must be two different species.
B) If they are unable to interbreed successfully, they must be two different species.
C) If they avoid each other completely, they must be two different species.
D) If they were genetically isolated for a long time, they must be two different species.
E) There's no way to determine whether they've become two separate species.