Over a two-week period, Jim offered Dwight a mint each time…

Over a two-week period, Jim offered Dwight a mint each time his computer made the “ding” sound after powering on. Dwight, although suspicious of Jim’s generosity, accepted the mint because he wanted to take any of Jim’s resources that he could. Each time he placed the mint in his mouth, experienced salivation. One day, Jim turned on his computer, making the “ding” sound. Dwight suddenly started to salivate.  Which of the following is the unconditioned stimulus in this example?