Which example BEST reflects a variable-ratio schedule of reinforcement?

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A variable-ratio schedule of reinforcement is one in which a response is reinforced after an unpredictable number of responses. This unpredictability makes the reinforcement highly effective at maintaining consistent behavior. Playing a slot machine is a classic example of this schedule, as the player is rewarded with a jackpot after an unpredictable number of plays.

Which Example BEST Reflects a Variable-Ratio Schedule of Reinforcement