2.3

Consider the time_slot relation. Given that a particular time slot can meet more than once in a week, explain why day and start_time are part of the primary key of this relation, while end_time is not.

The attributes day and start_time are part of the primary key since a particular class will most likely meet on several different days and may even meet more than once in a day. However, end_time is not part of the primary key since a particular class that starts at a particular time on a particular day cannot end at more than one time.