Six different presentations have been scheduled in six time…

Six different presentations have been scheduled in six time slots:   8:00-9:00, 9:00-10:00, 10:00-11:00, 11:00-12:00, 2:00-3:00, and 3:00-4:00.   Suppose you are allowed to attend four presentations.  How many different choices do you have if you attend exactly four presentations?