Question: which river creates a natural boundary for the louisiana purchase of 1803
[{'answer': 'Red River', 'provenance': [{'wikipedia_id': '994488', 'title': 'Neutral Ground (Louisiana)', 'start_paragraph_id': 3, 'start_character': 215, 'end_paragraph_id': 3, 'end_character': 224, 'bleu_score': 1.0, 'section': 'Section::::Background.\n'}]}, {'provenance': [{'wikipedia_id': '994488', 'title': 'Neutral Ground (Louisiana)', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 548, 'bleu_score': 0.977897140678983, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 16403075005493427117}, 'section': 'Section::::Background.\n'}]}]