You are afraid of tunnels. Your therapist first helps you to deeply relax and then asks you to ...

You are afraid of tunnels. Your therapist first helps you to deeply relax and then asks you to imagine a distant tunnel. When you can stay relaxed while visualizing this image, the therapist then asks you to imagine nearing the entrance, and finally entering the tunnel. Eventually, when you can remain relaxed while imagining all of this, the therapist takes you through a short tunnel. This list of increasingly fear-provoking situations is known as ____.
 
  a. assertiveness training
  b. stress-inoculation therapy
  c. a token economy program
  d. a desensitization hierarchy