Question: when did las vegas become a gambling city
[{'answer': '1931', 'provenance': [{'wikipedia_id': '2087269', 'title': 'History of Las Vegas', 'start_paragraph_id': 3, 'start_character': 339, 'end_paragraph_id': 3, 'end_character': 343, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '2087269', 'title': 'History of Las Vegas', 'start_paragraph_id': 3, 'start_character': 88, 'end_paragraph_id': 3, 'end_character': 754, 'bleu_score': 0.7348752910909241, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 14844498452928528055}, 'section': 'Section::::Abstract.'}]}]