Question: what was the longest lasting military engagement of ww2
[{'answer': 'The Battle of Verdun', 'provenance': [{'wikipedia_id': '51497', 'title': 'Battle of Verdun', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 20, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '51497', 'title': 'Battle of Verdun', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 851, 'bleu_score': 0.626219525965904, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11211324594175890670}, 'section': 'Section::::Abstract.'}]}]