Question: who was the longest serving prime minister of britain
[{'answer': 'Robert Walpole', 'provenance': [{'wikipedia_id': '6092765', 'title': 'List of Prime Ministers of the United Kingdom by length of tenure', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 14, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '6092765', 'title': 'List of Prime Ministers of the United Kingdom by length of tenure', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 158, 'bleu_score': 0.929712191543383, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 6124936824576273075}, 'section': 'Section::::Abstract.'}]}]