Question: when was the last year quarters were made of silver
[{'answer': '1964', 'provenance': [{'wikipedia_id': '32150', 'title': 'Quarter (United States coin)', 'start_paragraph_id': 37, 'start_character': 185, 'end_paragraph_id': 37, 'end_character': 189, 'bleu_score': 1.0, 'section': 'Section::::Silver series.\n'}]}, {'provenance': [{'wikipedia_id': '32150', 'title': 'Quarter (United States coin)', 'start_paragraph_id': 37, 'start_character': 0, 'end_paragraph_id': 37, 'end_character': 189, 'bleu_score': 0.9075772733559511, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8535218044196394442}, 'section': 'Section::::Silver series.\n'}]}]