Populations That Were Once Considered Separate Species Are Sometimes Reassigned

Populations that were once considered separate species are sometimes reassigned to the same species because:

A) it was discovered that they can produce viable and fertile offspring.
B) they have evolved to look different from each other.
C) they are no longer capable of interbreeding.
D) they have evolved to look the same.
E) they have been found to be genetically identical.