While Training His Dog, Mark First Says the Word "Sit

Multiple Choice

While training his dog, Mark first says the word "sit" and once the dog sits, he gives her a treat. Immediately after this, he says, "Good dog!" He repeats the same process many times, and each time his dog sits after listening to his command. In this example, which of the following is the primary reinforcer?

A) The word "sit"
B) Mark
C) The words "good dog"
D) The treat