Serotonin Levels Are Low in Those with Eating Disorders and in Those

Serotonin levels are low in those with eating disorders and in those with obsessive-compulsive disorder and depression. This means that:

A) low serotonin causes all three disorders.
B) all the disorders cause serotonin to decrease.
C) there is a relationship, but no evidence of causation.
D) if we raise serotonin levels, we will cure eating disorders.