What Would Be a Rationale as to Why the Number

What would be a rationale as to why the number of people affected during their early 20's with bipolar disorder is underestimated.

A) Early presenting symptoms are diagnosed as an attention disorder. The initial symptom may also be depression but when mania is experienced, reclassification occurs from a unipolar to a bipolar disorder.
B) Early presenting symptoms can present as a hyperactivity disorder, and later depression is diagnosed.
C) Most people do not seek treatment during the early stages.
D) Clinicians have difficulty differentiating unipolar disorder from bipolar disorder at this age.