John is in line at a retail clothing store. He is learning to stand in line and wait for a cashier to open. There are 4 cashiers and as soon as one lights up the "available" sign, he immediately goes to pay for his items. John's ability to identify between cashiers with a lit 'available' sign and those without and respond correctly would be the result of: 1. a. SD 2. b. Generalization 3. c. Stimulus Control 4. d. Discrimination training