\begin{tabular}{l} In Pavlov's research, dogs learned that the sound of a metronome predicted the \\ arrival of food, so the dogs salivated at the sound of the metronome. In this \\ research, classical conditioning occurred in part because the ___ elicited a(n) \\ response (salivation). \\ conditioned stimulus (food); conditioned \\ unconditioned stimulus (metronome); conditioned \\ unconditioned stimulus (food); unconditioned \\ \hline conditioned stimulus (metronome); unconditioned \end{tabular}

In Pavlov's research, the dogs learned to associate the sound of the metronome with the arrival of food. This process is a classic example of classical conditioning. To fill in the blank in the statement, we need to identify the correct terms: 1. **Unconditioned Stimulus (US)**: This is a stimulus that naturally and automatically triggers a response without any conditioning. In this case, the food is the unconditioned stimulus because it naturally causes the dogs to salivate. 2. **Conditioned Stimulus (CS)**: This is a previously neutral stimulus that, after being paired with the unconditioned stimulus, comes to trigger a conditioned response. In this case, the metronome is the conditioned stimulus because it was initially neutral but became associated with the food. 3. **Unconditioned Response (UR)**: This is the natural response to the unconditioned stimulus. Here, salivation in response to food is the unconditioned response. 4. **Conditioned Response (CR)**: This is the learned response to the conditioned stimulus. In this case, salivation in response to the metronome is the conditioned response. Given this information, the correct completion of the statement is: "In this research, classical conditioning occurred in part because the **unconditioned stimulus (food)** elicited a(n) response (salivation)." Thus, the answer is: **unconditioned stimulus (food)**.