This semester Professor Popkin will commute to Montreal by plane, leaving every Tuesday and returning to Toronto every Friday. She instructs her travel agent to schedule for her one round-trip that departs from Toronto on the first Tuesday and returns to Toronto on the last Friday of the semester and additional round-trips that depart from and return to Montreal for each weekly commute in between. Which one of the following, if true, most helps to explain Professor Popkin's instructions to her travel agent regarding the grouping of her flights into round-trips? ["If all Professor Popkin's tickets are paid for at one time, she will be eligible for a frequent-traveler discount.", "Professor Popkin's round-trip tickets are least expensive if each trip is scheduled so that at least one Saturday night intervenes between the departing flight and the returning flight.", 'In order for Professor Popkin to receive the lowest possible airfare, each of her round trips must be paid for at least seven days in advance of the departing flight.', 'A ticket for a round-trip in which the departing flight and the returning flight are separated by several months will cost Professor Popkin more than a ticket for a round-trip in which they are separated by less than one week.']