The correct answer is D. (Judy discovered a shark’s tooth after several hours of searching for one on the beach). A variable-ratio schedule is a schedule in which a reward comes after a certain number of times a person performs a certain behavior. Also, the number of times that the person has to perform a behavior between the rewards differs each time. Judy was searching, possibly digging through the sand for certain amount of times before she found the tooth. However, there is no way of knowing how many times will she have to do that before she finds another one (the tooth is a reward in this example).