What does research reveal about the role genetic factors play in unipolar depression?
a) Twin studies have disproven the theory that genetic factors play a role in unipolar depression.
b) Genetic factors are believed to play a role, but the evidence consists entirely of family pedigree studies and twin studies.
c) A few chromosomes are suspected of harboring genes related to depression, but no specific genes have been identified.
d) Researchers have found genes associated with unipolar depression on most of the body's chromosomes.
