Question: which world war 1 military tactic resulted in a prolonged stalemate
[{'answer': 'trench warfare', 'provenance': [{'wikipedia_id': '182259', 'title': 'Trench warfare', 'start_paragraph_id': 1, 'start_character': 248, 'end_paragraph_id': 1, 'end_character': 262, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '182259', 'title': 'Trench warfare', 'start_paragraph_id': 156, 'start_character': 0, 'end_paragraph_id': 156, 'end_character': 207, 'bleu_score': 0.9402297675816911, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 18440330663305304134}, 'section': 'Section::::World War I – breaking the stalemate.\n'}]}]