Question: who was the longest serving supreme court justice
[{'answer': 'William O. Douglas', 'provenance': [{'wikipedia_id': '3614375', 'title': 'List of United States Supreme Court Justices by time in office', 'start_paragraph_id': 1, 'start_character': 433, 'end_paragraph_id': 1, 'end_character': 451, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]