Consider an experiment in which a pigeon is placed in a Skinner box with a button on the wall. The cage contains a small green light that is turned on and off intermittently throughout the day. When the pigeon pecks the button when the green light is on, the behavior is reinforced with food. When it pecks the button when the green light is off, the behavior is not reinforced. The pigeon soon learns that button pecking will be rewarded only when the green light is on. In this scenario, the green light is a(n) .