Question: when did the usd stop being backed by gold
[{'answer': 'August 15 , 1971'}, {'provenance': [{'wikipedia_id': '3606600', 'title': 'Nixon shock', 'start_paragraph_id': 13, 'start_character': 0, 'end_paragraph_id': 13, 'end_character': 388, 'bleu_score': 0.5320469155115021, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 15317046436697447353}, 'section': 'Section::::Event.\n'}]}]