Lexie lives at sea level, but her family rented a vacation cabin high in the Rocky Mountains for two weeks. She noticed a difference in her breathing. An image of a mountain with three circles going up the side of it. The lowest circle is filled with many, smaller circles. The middle circle has fewer circles that are farther apart. The highest circle has the fewest circles that are the farthest apart.

What do the molecules in the circles model regarding altitude?
A. Air pressure increases at higher altitudes.
B. Air density decreases at higher altitudes.
C. There is a greater gravitational pull on air molecules at higher altitudes.
D. Temperature has a decreasing effect on air molecules at higher altitudes.