A balloon is filled with cold air and placed in a warm room. It is NOT in thermal equilibrium with the air of the room until
(1) it rises to the ceiling.
(2) it sinks to the floor.
(3) it stops expanding.
(4) it starts to contract.
When the temperature inside and outside the balloon becomes same, that is, the air inside and outside the balloon are in thermal equilibrium, then the balloon stops expanding.
Hence, the correct option is (3).
