The results of studies investigating the role of biological factors suggest that depression is caused by select one: a. variations in the serotonin transporter gene b. increases in polypeptide y in the brain c. decreases in the levels of gastric ghrelin d. unusual elevations in both dopamine and glutamate levels

The results of studies investigating the role of biological factors suggest that depression is caused by variations in the serotonin transporter gene.
Serotonin transporter gene is a gene that controls how many serotonin re-uptake transporters will be produced. It has been shown that mutations in this gene are linked to depression (depression has been linked to low levels of serotonin).

idk but if i had to take an educated guess i’d say a c d