Question: when did the nba start best of 7
[{'answer': '1958', 'provenance': [{'wikipedia_id': '12106552', 'title': 'NBA playoffs', 'start_paragraph_id': 10, 'start_character': 570, 'end_paragraph_id': 10, 'end_character': 574, 'bleu_score': 1.0, 'section': 'Section::::History.\n'}]}, {'provenance': [{'wikipedia_id': '12106552', 'title': 'NBA playoffs', 'start_paragraph_id': 10, 'start_character': 0, 'end_paragraph_id': 10, 'end_character': 622, 'bleu_score': 0.9513840121740441, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9469202125840707040}, 'section': 'Section::::History.\n'}]}]