In the Course of Studying Plant Communities Living on an Elevational

In the course of studying plant communities living on an elevational gradient in Rocky Mountain National Park, researchers observed that a plant species growing in the park appears to actually be three species. The low elevation (LE) species is more clearly distinct from the high elevation (HE) species, but the middle elevation (ME) species has characteristics of both. After taking seeds from all three populations back to their greenhouse for a reproductive study, the researchers discovered that ME plants could not successfully pollinate HE plants, but ME did produce sterile seeds with the LE plants. Which statement best describes the reproductive isolating mechanisms between these populations?

A) ME and HE are isolated through postzygotic mechanisms, while ME and LE are isolated through prezygotic mechanisms.
B) ME and HE are isolated through prezygotic mechanisms, while ME and LE are isolated through postzygotic mechanisms.
C) All three populations appear to be isolated through postzygotic mechanisms.
D) All three populations appear to be isolated through prezygotic mechanisms.