A. they have to pay additional money to get the seats they want
B. they have to pay different prices based on the day of purchase
C. they are confused by the advance purchase patterns
D. there are too many combinations of fares offered by airlines

A. To review what students know about volcanic activity.
B. To demonstrate the use of a new measurement device.
C. To explain the answer to an examination question.
D. To provide background for the next reading assignment.