Question: when did the us currency stop being backed by gold
[{'answer': '1971', 'provenance': [{'wikipedia_id': '1978363', 'title': 'History of the United States dollar', 'start_paragraph_id': 14, 'start_character': 1004, 'end_paragraph_id': 14, 'end_character': 1008, 'bleu_score': 1.0, 'section': 'Section::::Early history.\n'}]}, {'provenance': [{'wikipedia_id': '1978363', 'title': 'History of the United States dollar', 'start_paragraph_id': 14, 'start_character': 0, 'end_paragraph_id': 14, 'end_character': 1008, 'bleu_score': 0.97132316035213, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 2170693328239583958}, 'section': 'Section::::Early history.\n'}]}]