Final answer:
The interval representing a business cycle expansion goes from the trough to the peak. Without seeing the actual figure, we deduce that intervals A to B and C to D likely represent expansions since these intervals would be part of the upward movement from a trough towards a peak, while intervals B to C and D to A would represent contractions.
Step-by-step explanation:
The interval that represents a business cycle expansion is typically understood as the phase from the trough, or the lowest point after a recession, to the peak, which is the highest point before the next recession. Based on the description and the information provided, a business cycle expansion is characterized by a period where the economy grows and moves towards its peak after hitting its lowest point.
Without the ability to view the actual figure this student refers to, we can utilize the information given about the business cycle: an expansion runs from trough to peak. If the points A to C represented a complete cycle, B to D would also likely represent a complete cycle, identifying A to B and C to D as expansion phases, since they would be coming out of a trough and moving towards a peak. Consequently, based on the general knowledge, intervals B to C would indicate periods of recession or contraction, where the economy declines from a peak and moves towards a trough.