Question: what are the major hubs for united airlines
[{'answer': "Chicago -- O'Hare"}, {'answer': 'Houston -- Intercontinental'}, {'answer': 'Newark', 'provenance': [{'wikipedia_id': '32307', 'title': 'United Airlines', 'start_paragraph_id': 77, 'start_character': 87, 'end_paragraph_id': 77, 'end_character': 93, 'bleu_score': 1.0, 'section': 'Section::::Cabin.:United premium transcontinental service.\n'}]}, {'answer': 'Denver', 'provenance': [{'wikipedia_id': '32307', 'title': 'United Airlines', 'start_paragraph_id': 102, 'start_character': 55, 'end_paragraph_id': 102, 'end_character': 61, 'bleu_score': 1.0, 'section': 'Section::::Corporate affairs.:Other facilities.\n'}]}, {'answer': 'San Francisco', 'provenance': [{'wikipedia_id': '32307', 'title': 'United Airlines', 'start_paragraph_id': 77, 'start_character': 113, 'end_paragraph_id': 77, 'end_character': 126, 'bleu_score': 1.0, 'section': 'Section::::Cabin.:United premium transcontinental service.\n'}]}, {'answer': 'Washington -- Dulles'}, {'answer': 'Los Angeles', 'provenance': [{'wikipedia_id': '32307', 'title': 'United Airlines', 'start_paragraph_id': 77, 'start_character': 98, 'end_paragraph_id': 77, 'end_character': 109, 'bleu_score': 1.0, 'section': 'Section::::Cabin.:United premium transcontinental service.\n'}]}]