Answer:
Refrigerator
Step-by-step explanation:
Think about it. Equilibrium is between 2 "places", and if you didn't open the fridge, there would be a buildup of cold air.
Because of that buildup, it would keep getting colder until it isn't possible to get cold anymore. That is equilibrium.
However, if you were to open the door for a very, very long time, you would lower the temp. inside the fridge, therefore causing for the room you are in to be slightly colder, and the fridge to decrease in cold. The fridge will still be slightly cold, but it won't be lukewarm if you leave it for 2 hours. (Unless it's broken)
That is equilibrium :)