A cat is classically conditioned to purr when it hears the refrigerator door open, because it contains the cat food. But the cat does not purr when it hears the pantry door open. The fact that the cat only purrs when the refrigerator door opens is most likely a result of

Conditioning.

Explanation:

So the cat seems to have picked up that food only comes out of the fridge and not out of the pantry. The cat probably has watched its humans open the panrtybut never gave the it any food so it now knows..."hey that only human food". But it probably has saw the humans get food out of the fridge and saw the pattern of the fridge = food.  The opposite story is true for my own cat, she would always rub against me when she heard the pantry open, but left me alone when the fridged opened, because she has learned food only comes out of the pantry.  

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