Question: when did nba playoffs change to 7 game series
[{'answer': '1947', 'provenance': [{'wikipedia_id': '12106552', 'title': 'NBA playoffs', 'start_paragraph_id': 8, 'start_character': 343, 'end_paragraph_id': 8, 'end_character': 347, 'bleu_score': 1.0, 'section': 'Section::::History.\n'}]}, {'provenance': [{'wikipedia_id': '12106552', 'title': 'NBA playoffs', 'start_paragraph_id': 23, 'start_character': 0, 'end_paragraph_id': 23, 'end_character': 613, 'bleu_score': 0.8627773444018211, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 3444979952424757343}, 'section': 'Section::::History.:Timeline.\n'}]}]