Read the Following Scenario and Answer the Question Below

Read the following scenario and answer the question below.
The western United States contains a series of tall mountain ranges that also extend far down into Mexico. The largest of these ranges are the Rockies and the Sierra Madres, but they are all part of the same mountain- building event that occurred many millions of years ago, and they all have similar features. Much of the vegetation on these mountaintops is the same, or very similar, and they are often referred to as the "Sky Islands" because of their similarity to each other and their isolation from the lower, drier areas surrounding them.
-There are groups of tropical birds, such as trogons and parrots, that are usually considered to live in Central America and the mountains of southern and central Mexico. Small populations of these birds migrate into the United States each year. It is probably true that _______________

A) these birds migrate to the United States in the winter because it is warmer here
B) these birds can live in the lower, drier elevations in the United States as well as in the mountain ranges
C) more of these birds are found in the taller mountains of Colorado than in the shorter mountains in southern Arizona, even if the relative area of the mountains is the same
D) more of these birds are found in the mountains of New Mexico and Arizona than in the mountains of Colorado, Utah, or Nevada
E) these birds have higher genetic diversity in the U.S. populations than in the populations in Central America