Dustin investigated the effect of increasing the number or "D" batteries in a circuit of loudness of a buzzer within the circuit. He created four identical circuits that each included a buzzer, and used 1, 2, 3 and 4 batteries in each of the four circuits. He listened to the sounds of the different buzzers and how they were louder and softer depending on the number of batteries used. Which variable was the responding variable, or dependent variable, in his experiment?