Two system are said to be in thermal equilibrium if and only if:
- there can be a heat flow between them even are at different temperatures
- there cannot be a heat flow between them even if they are at different temperatures
- there is no heat flow between them
- their temperature are slightly different
Two physical systems are in thermal equilibrium if no heat flows between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics.
The correct option is C.