Question: where is the heart's natural pacemaker located
[{'answer': 'sinoatrial ( SA ) node', 'provenance': [{'wikipedia_id': '233429', 'title': 'Cardiac pacemaker', 'start_paragraph_id': 1, 'start_character': 527, 'end_paragraph_id': 1, 'end_character': 550, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '233429', 'title': 'Cardiac pacemaker', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 616, 'bleu_score': 0.980097014099621, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13501199198924276935}, 'section': 'Section::::Abstract.'}]}]