Answer:
B. Warming air expands, becomes less dense, and rises. As the air rises, it pushes cooler, more dense air down.
Explanation:
The options D and A can be eliminated because when a gas expands without gaining additional mass, it will become less dense; when a gas contracts without losing mass, it becomes more dense. Therefore, these two options conflict with the idea of density and cannot be true.
Option B is superior to Option C because convection currents are typically portrayed as beginning with the rise of warm air, which displaces/is replaced by cooler, dense air from the upper atmosphere. While Option C is similar, it describes the opposite series of events.
