Jack reports being in a wonderful mood. He has been very active at work lately, even working far into the night, as he seems to need only a few hours of sleep at night. He is very talkative and quickly moves from one topic to another, describing a scheme he has for making a fortune in the stock market. These behaviors represent a change from Jack's normal behaviors and have caused severe functional impairment. He reports no previous periods like this. Which of the following diagnoses would best fit Jack?
a) Overanxious disorder.
b) Dysthymic disorder.
c) Bipolar disorder.
d) Cyclothymic disorder.