What are the levels of serotonin in the body when someone is feeling depressed, and how can depression be treated?

a. Serotonin levels are low, and by increasing serotonin, the patient will have an increase in happy feelings.
b. Serotonin levels are high, and decreasing serotonin, the patient will have an increase in happy feelings.
c. Serotonin levels are constant, and by decreasing serotonin serotonin, the patient will have an increase in happy feelings.
d. Serotonin levels are fluctuating, and by increasing serotonin, the patient will have an increase in happy feelings.