Yolanda manages a Best Sleep Inn along an interstate highway. She knows from experience that 5 to 10 last-minute customers will call after 8 p.m. each evening looking for a room and asking the price. Yolanda has empowered her staff to offer discounts when the motel is largely vacant and to quote the standard price when the motel is close to full. She knows her service is __________, meaning that if no one stays in the room, it generates no revenue that evening.