Question: what people group established an empire in the valley of mexico
[{'answer': 'The Aztec', 'provenance': [{'wikipedia_id': '19222', 'title': 'History of Mexico', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 9, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '19222', 'title': 'History of Mexico', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 128, 'bleu_score': 0.9067110266941041, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4665630227506077012}, 'section': 'Section::::Abstract.'}]}]