What should biologists do when two similar organisms that were originally thought to be separate species are found to interbreed freely and produce normal offspring whenever they live in the same habitat?

A) Develop a new name for the hybrids.
B) Retain the two species' names if the organisms have very different appearances.
C) Separate the two groups until they become dissimilar enough to prevent interbreeding.
D) Consider both types to be a single species.