Final answer:
When air or water is heated, its density decreases, and the air or water rises, creating currents.
Step-by-step explanation:
The correct explanation for the movement of air and water in currents is option A: When air or water is heated, its density decreases and the air or water rises.
This is due to the fact that as the temperature of fluids increases, they expand and become less dense. When air or water is heated, the molecules move faster and spread out, causing the density to decrease. As a result, the less dense air or water rises, creating currents.
For example, the buoyancy of warm air and water causes ocean currents and large-scale atmospheric circulation, transferring hot air from the tropics toward the poles and cold air from the poles toward the tropics.