Research Indicates That Serotonin Is Important in Anxiety Disorders

Research indicates that serotonin is important in anxiety disorders. Partial evidence for this is found in biochemical challenge studies in which

A) participants' serotonin levels are manipulated and their anxiety levels monitored.
B) serotonin levels are monitored to examine how decreased levels are related to decreased anxiety.
C) patients with anxiety disorders are studied by examining their lack of ability to produce serotonin in response to a threat.
D) participants with anxiety disorders are presented with feared objects and their serotonin levels monitored.