Which historic airline merger in 2010 created what was then the world's largest airline by fleet size and passenger traffic?

In the world of transportation, corporate mergers can dramatically reshape the industry landscape. One particularly significant merger in aviation history created what was, at the time, the world's largest airline. This merger represented a major consolidation in the airline industry and set off a wave of similar mergers among competitors. Test your knowledge about this transformative business move that fundamentally changed the airline industry and created a transportation powerhouse.

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United Airlines and Continental Airlines
Delta Air Lines and Northwest Airlines
American Airlines and US Airways
Southwest Airlines and AirTran Airways