Question: during which phase of the cardiac cycle does the first heart sound occur
[{'answer': 'isovolumic contraction', 'provenance': [{'wikipedia_id': '2525843', 'title': 'Cardiac cycle', 'start_paragraph_id': 3, 'start_character': 583, 'end_paragraph_id': 3, 'end_character': 605, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]