Question: when were u boats first used in ww1
[{'answer': '5 September 1914', 'provenance': [{'wikipedia_id': '31888', 'title': 'U-boat', 'start_paragraph_id': 8, 'start_character': 3, 'end_paragraph_id': 8, 'end_character': 19, 'bleu_score': 1.0, 'section': 'Section::::World War I (1914–1918).\n'}]}, {'provenance': [{'wikipedia_id': '31888', 'title': 'U-boat', 'start_paragraph_id': 8, 'start_character': 0, 'end_paragraph_id': 8, 'end_character': 251, 'bleu_score': 0.597628365683986, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9243701141386010048}, 'section': 'Section::::World War I (1914–1918).\n'}]}]