Question: when was the last time a us submarine sunk
[{'answer': '21 February 1946', 'provenance': [{'wikipedia_id': '416011', 'title': 'List of lost United States submarines', 'start_paragraph_id': 18, 'start_character': 332, 'end_paragraph_id': 18, 'end_character': 348, 'bleu_score': 1.0, 'section': 'Section::::During World War II.:Additional incidents.\n'}]}, {'provenance': [{'wikipedia_id': '416011', 'title': 'List of lost United States submarines', 'start_paragraph_id': 18, 'start_character': 0, 'end_paragraph_id': 18, 'end_character': 462, 'bleu_score': 0.8695036301370711, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 225719552370803249}, 'section': 'Section::::During World War II.:Additional incidents.\n'}]}]