Classical Conditioning Example

In the study of classical conditioning, researchers have found that certain stimuli can elicit involuntary responses from organisms. A classic example of this process is seen in Pavlov's experiments with dogs where he conditioned them to salivate at the sound of a bell. In a similar experiment, a researcher pairs the sound of a metronome with the presentation of food to a group of rats. After several pairings, the rats begin to salivate at the sound of the metronome alone. This phenomenon illustrates which of the following concepts related to classical conditioning?