Rex, Liz's dog, runs towards Liz whenever he hears the electric tin opener being used.
Providing food to Rex could be seen as positive reinforcement because it would add a/n (a) stimulus that (b) the likelihood of the behaviour of running towards Liz at the sound of the electric tin opener.
Yelling at Rex could be seen as positive punishment because it wouId add a/n (c) stimulus that (d) the likelihood of the behaviour of running towards Liz at the sound of the electric tin opener.