In pavlov's early classical conditioning experiments, food was the and a bell which was originally a neutral stimulus became a when paired with the food.

The answer is unconditioned stimulus and conditioned stimulus, respectively.

Explanation:

In classical conditioning, a conditioned stimulus (the sound of the bell) is associated with an unrelated consequence, called unconditioned stimulus. This produces an involuntary reaction called conditioned response.

The unconditioned stimulus is usually related to physiological needs, such as food or the avoidance of pain.

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