Question: when was the us dollar backed by silver
[{'answer': '1878 to 1964', 'provenance': [{'wikipedia_id': '1978363', 'title': 'History of the United States dollar', 'start_paragraph_id': 36, 'start_character': 83, 'end_paragraph_id': 36, 'end_character': 95, 'bleu_score': 1.0, 'section': 'Section::::Silver standard.\n'}]}, {'provenance': [{'wikipedia_id': '1978363', 'title': 'History of the United States dollar', 'start_paragraph_id': 36, 'start_character': 0, 'end_paragraph_id': 36, 'end_character': 451, 'bleu_score': 0.9735040436119741, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8474258131782969107}, 'section': 'Section::::Silver standard.\n'}]}]