Peter and Luke Are Identical Twins Who Are Juniors at the Same

Peter and Luke are identical twins who are juniors at the same college. Throughout their college years, both have been treated for major depressive disorder. Based on what researchers have found from twin studies, we know that it is probable that Peter and Luke:

A) have abnormal brain structures, such as smaller frontal lobe and enlarged ventricles.
B) have a deficiency in the neurotransmitters norepinephrine and serotonin.
C) had a viral infection during prenatal development or shortly after birth.
D) have inherited a genetic predisposition for major depressive disorder.