Kino's parents gave her four-year-old son, lan, a noisemaker to celebrate his birthday. Since receiving the gift, lan has played with his noisemaker everyday. Desperate for some peace and quiet, kino gives lan a lollipop, which takes lan's attention away from the toy and gives kino some peace and quiet. However, once the lollipop is gone, lan returns to his noisemaker. Kino gives lan another lollipop. In this example, what kind of reinforcement process is taking place?