Which Research Finding Confirms a Role for Serotonin Dysfunction in Mood

Which research finding confirms a role for serotonin dysfunction in mood disorders?

A) Decreased numbers of postsynaptic 5-HT₂ receptors are found in depressed patients.
B) Low 5-HIAA turnover is found in depressed individuals.
C) Rapid tryptophan depletion in treated depressed patients improves symptoms.
D) Challenge studies indicate that the 5-HT receptors in depressed individuals are highly sensitive.