10. Garth has been taught to say "hello" whenever he encounters a staff member on his way to the cafeteria. In order to help Garth perform this behavior, staff members wave their hand when they see him enter the cafeteria. Hand waving is an example of a(n): (CAEP 1.1, 1.2, 1.3, 1.5)

a. Cognitive mediator

b. Discriminative stimuli

c. Isolated behavior

d. Contextual variation