Charlie’s dog jumps up on the legs of his guests as they enter his apartment. The guests always pet the dog. Charlie decides to address this problem behavior using a technique he learned in his behavior analysis class. Each day, when he enters his apartment and his dog jumps up on his legs, he does not pet the dog (extinction). Instead, he leads the dog 10 feet away, to a carpet square in the corner, and pets the dog enthusiastically when it sits on the square. Charlie practices this every day, when he arrives home. About two weeks into this training, Charlie comes home to find his dog already sitting on his carpet square as Charlie walks into the room. Charlie proudly reinforces this behavior. Later, when some friends come over to watch a movie, the dog runs to the carpet square as the guests arrive. Charlie instructs his friends to reinforce this behavior. What technique did Charlie use to teach his dog to sit on the carpet square, instead of jumping on the legs of his guests?