The Best Evidence That There Is a Difference Between Bipolar

The best evidence that there is a difference between bipolar affective disorder and unipolar depression is that

A) the age of onset differs between the two disorders.
B) there is a sex difference in the incidence of these disorders.
C) lithium is effective in treating only bipolar disorder.
D) chlorpromazine is only effective in treating unipolar depression.
E) serotonin reuptake inhibitors are effective for bipolar disorder but not unipolar depression.