Question: who has the most back to back nba championships
[{'answer': 'Boston Celtics', 'provenance': [{'wikipedia_id': '16795291', 'title': 'List of NBA champions', 'start_paragraph_id': 3, 'start_character': 372, 'end_paragraph_id': 3, 'end_character': 386, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]