Question: when were horses introduced to the united states
[{'answer': '1538', 'provenance': [{'wikipedia_id': '46694871', 'title': 'Horses in the United States', 'start_paragraph_id': 9, 'start_character': 258, 'end_paragraph_id': 9, 'end_character': 262, 'bleu_score': 1.0, 'section': 'Section::::Extinction and return.\n'}]}, {'provenance': [{'wikipedia_id': '46694871', 'title': 'Horses in the United States', 'start_paragraph_id': 9, 'start_character': 0, 'end_paragraph_id': 9, 'end_character': 569, 'bleu_score': 0.9776501033621461, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4420705378630258901}, 'section': 'Section::::Extinction and return.\n'}]}]