Thermal energy tends to flow as heat

A. from a system of lower energy to a system of higher energy.

B. only between systems that have equal energy.

C. only between systems that have equal temperatures.

D. from a system of higher energy to a system of lower energy.

D. from a system of higher energy to a system of lower energy; this is due to the zeroth law of thermodynamics, which states that when two objects are placed in contact heat (energy) is transferred from one to the other until they reach the same temperature (are in thermal equilibrium).

D. Thermal energy always flows from the warmer to the cooler one because the higher energy particles transfer energy to the lower energy ones as they collide