Pets who run toward the sound of an electric can opener because it signals the arrival of their canned food, even if there is no food present, illustrate what?

Classical conditioning

Explanation:

In psychology, classical conditioning is a method of learning by pairing stimuli. Usually, first one stimulus causes a natural answer (the unconditioned stimulus causes an unconditioned answer), then this unconditioned stimulus is paired with another stimulus that doesn't cause an answer by itself  at first but after some repetitions, the new stimulus (conditioned stimulus) causes the same answer from the unconditioned stimulus and this answer becomes the conditioned answer.

In this example, pets have learned to pair the sound of the electric can opener with the arrival of their canned food so they have learned to respond to this sound and therefore, they run toward the sound of the can opener even if there is no cat food present. The can opener would be the conditioned stimulus which has been paired with the food (unconditioned stimulus) and has created the response of the pets running toward this sound (Conditioned answer). Therefore this is an example of classical conditioning.  

the historians cited in the article all agree with one another about the cause of the civil war. despite coming from different backgrounds and cultures, the historians agree that slavery was the root cause of the civil war. the fact that all these historiansgenerally agree with one another makes their shared viewpoints more reliable and valid, especially because the historians have different backgrounds and all work for the well-respected institutions of howard university, clemson university, and the university of colorado.

no because it could make the world a terrible place to live in.