Sally Has Been Suffering from Depression and Finally Decides to Seek

Sally has been suffering from depression and finally decides to seek help from a clinical psychologist. After a couple of months of therapy, Sally starts to recover from depression. However, her improvement really isn't due to the therapy she has received from her therapist, but instead is a product of Sally's expectation that psychotherapy is
Supposed to be effective and therefore she should be getting better. This example is best considered as an example of:

A) social desirability.
B) the double-blind effect.
C) the placebo effect.
D) an experimenter expectancy effect.