What happened to the greenish warbler birds as they migrated from the southern portion of their range up around the tibetan plateau?
a. they evolved separately as they moved north and became two separate species.
b. the original species died off, but the ones that were able to adapt to a northern climate evolved into a new species.
c. as a whole the species was able to adapt to a colder climate, but no significant changes occurred.
d. they were unable to adapt and evolve to ensure their survival and became extinct.
a?

The correct answer is option a. "They evolved separately as they moved north and became two separate species".

Explanation:

The greenish warbler is a species of birds that lives and breeds  in northeastern Europe and parts of Asia with a temperate temperature. When the greenish warbler birds migrated from the southern portion of their range up around the Tibetan Plateau, they evolved separately as they moved north and became two separate species. The subspecies of the greenish warbler are hard to distinguish, however they are known as the "green warbler" and has a yellowish color at the supercilium, throat, breast and faint wing bar.

igneous rocks which formed by the crystallization of magma at the surface of the earth called extrusive rocks. they are characterized by fine-grained textures because the rapid cooling at or near the surface did not provide enough time for large crystals to grow.