Task: Question Generation
Input (English): Houston was the headquarters of Continental Airlines until its 2010 merger with United Airlines with headquarters in Chicago; regulatory approval for the merger was granted in October of that year. Bush Intercontinental became United Airlines' largest airline hub. The airline retained a significant operational presence in Houston while offering more than 700 daily departures from the city. In early 2007, Bush Intercontinental Airport was named a model "port of entry" for international travelers by U.S. Customs and Border Protection.
Output (English): Where is United Airlines' largest air hub?