What is the {airport} for {airline hub} of {Delta Air Lines}
Which airport is the hub of Delta Air Lines?
 select distinct ?obj where { wd:Q188920 wdt:P113 ?obj . ?obj wdt:P31 wd:Q1248784 } 
select distinct ?obj where { ?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#subject> <http://wikidata.dbpedia.org/resource/Q188920> . ?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#predicate> <http://www.wikidata.org/entity/P113> . ?statement <http://www.w3.org/1999/02/22-rdf-syntax-ns#object> ?obj . ?obj <http://www.wikidata.org/entity/P31> <http://wikidata.dbpedia.org/resource/Q1248784> } 
Delta Airlines has it's hub located in what major airport?