An Outcome Study Includes Three Groups of Depressed Clients

An outcome study includes three groups of depressed clients. Group I receives an SSRI along with cognitive therapy. Group II receives only an SSRI. Finally, Group III receives a placebo. All groups complete a paper-and-pencil inventory of depressive symptoms both at the outset of the study and at its conclusion. Depressive symptoms are reduced among all three groups. However, the reduction is greater for Group I than for Group II, and for Group II as compared to Group III. Which of the following conclusions is INCORRECT

A) The difference between Groups I and II reflects a nonspecific treatment effect.
B) The contribution of natural improvement to the present results remains unknown.
C) Cognitive therapy is mainly responsible for the fact that the improvement in Group I was greater than that in Group II.
D) Placebo effects may be partly responsible for Group I's improvement.