A Client Comes to a Therapist Complaining of a Great

A client comes to a therapist complaining of a great fear (phobia) of heights. The therapist first has the individual describe and think about increasingly high places. Next, he goes with the client to the first floor of a tall building, all the while encouraging and reassuring. They spend time looking out the window until the client is comfortable, then they go up one floor. Gradually, they work their way up to the top floor of a very high skyscraper. The treatment method the therapist is employing is called

A) flooding.
B) deconditioning.
C) reconditioning.
D) systematic desensitization.
E) unconditioning.