Suppose That a Therapist Notices Several Clients with Anxiety Disorders

Suppose that a therapist notices several clients with anxiety disorders also demonstrate symptoms consistent with major depressive disorder.Why should she be cautious in making the assumption that experiencing an anxiety disorder causes one to experience major depressive disorder?

A) Her observations may not be replicated by other researchers.
B) The relationship is probably in the opposite direction; having major depressive disorder causes one to experience anxiety disorders.
C) The relationship may really be the result of a third variable such as lack of personal control.
D) The observed relationship creates an unfalsifiable research question.