Ring Species Such as the Greenish Warbler Complicate the Biological

Ring species such as the greenish warbler complicate the biological species concept because:

A) at least some of their populations are reproductively isolated from one another but can still exchange genetic material.
B) at least some of their populations overlap spatially, but individuals in those populations do not interbreed with one another.
C) individuals in at least some populations interbreed with individuals from other species.
D) ring species comprise populations that are in the process of allopatric speciation.
E) None of the other answer options is correct.