When Maddie checked in at the Happy Airline ticket counter for her five-hour economy-class flight to California, the customer service agent explained that hot meals and complimentary cocktails would only be served to business-class and first-class passengers. For that reason, the Happy Airline agent suggested that Maddie pay a small fee to move from economy-class to business-class for the flight. This is an example of Group of answer choices A. a trade-in allowance B. affiliate marketing C. comparison shopping D. Up-selling