Which of the following best defines the condition of thermal equilibrium between two systems?
(a) The systems have identical masses and internal energies, eliminating any heat transfer.
(b) The systems are at the same temperature but separated by an insulating barrier, so thermal equilibrium is maintained.
(c) The systems are in thermal contact and at the same temperature, so no net energy is transferred between them.
(d) The systems have different temperatures, but heat flows equally in both directions, resulting in zero net transfer.