A science teacher set up a fish tank in her classroom to bring science to life. While cleaning the fish tank, a small piece of a plant broke away from the stem and floated upward and in the opposite direction of the heater. Upon reaching the opposite end of the tank, the plant piece gradually sank. It then floated back to the heater, where it rose once again. This movement repeated until the plant piece was removed from the fish tank. As water cools, it becomes more dense. As water reached the surface of the tank, it became cooler and began to sink. As the water sank, it heated again and moved upward again.
The plant piece cycled around the tank with the moving water. This cycling of cool and warm water is an example of
A. convection.
B. conduction.
C. expansion.
D. radiation.