Which promotion exemplifies the use of a variable-ratio schedule of reinforcement?

Question

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A variable-ratio schedule of reinforcement involves reinforcing a behavior after a random number of times it is performed. In this case, the "You are a winner" promotion randomly reinforces the behavior of purchasing a beverage, as only one-twelfth of the coffee lids have the winning message. This creates a variable-ratio schedule of reinforcement, as patrons never know when they may receive a free beverage, but the possibility is always there.

Which Promotion Exemplifies the Use of a Variable-Ratio Schedule of Reinforcement