Julie has a fear of riding in elevators. she goes to a therapist who first has her stand outside an elevator with him, and he talks to her until she is calm. then, the two of them stand outside the elevator with the doors open. then, they stand in it but do not ride in it. finally, they ride in the elevator. at each stage, the therapist waits until julie is completely relaxed before moving on to the next stage. this is an example of