Ms. Avila is the CEO for a social media firm and stays inside her office most of the time. At the same time every hour, however, she steps outside her office to see what her employees are doing. The employees have learned to work especially hard during the five minutes before and while she is outside her office. Their work pattern is typical of responses that are reinforced on a ________ schedule. a. fixed-interval b. variable-ratio c. variable-interval d. fixed-ratio