Question: "What indicates there must be at least one other airport in Australia?" Context: "Melbourne has four airports. Melbourne Airport, at Tullamarine, is the city's main international and domestic gateway and second busiest in Australia. The airport is home base for passenger airlines Jetstar Airways and Tiger Airways Australia and cargo airlines Australian air Express and Toll Priority; and is a major hub for Qantas and Virgin Australia. Avalon Airport, located between Melbourne and Geelong, is a secondary hub of Jetstar. It is also used as a freight and maintenance facility. Buses and taxis are the only forms of public transport to and from the city's main airports. Air Ambulance facilities are available for domestic and international transportation of patients. Melbourne also has a significant general aviation airport, Moorabbin Airport in the city's south east that also handles a small number of passenger flights. Essendon Airport, which was once the city's main airport also handles passenger flights, general aviation and some cargo flights." Answer:
second busiest