There are 16 students giving final presentations in your history course. (a) Three students present per day. How many presentations orders are possible for the first day? (b)Presentation subjects are based on the units of the course. Unit B is covered by three students, Unit C is covered by 5 students, and Units A and D are each covered by four students. How many presentation orders are possible when presentations on the same unit are indistinguishable from each other?