One Piece of Evidence Suggesting That Bipolar and Major Depressive

One piece of evidence suggesting that bipolar and major depressive disorder are two different entities is that

A) people are much more severely depressed with major depressive disorder.
B) people are more apt to commit suicide with major depressive disorder.
C) bipolar disorder can often be treated with lithium,which does not help people with major depressive disorder.
D) major depressive disorder has a familial component,but bipolar disorder does not.