Question: when did they stop using silver in half dollars
[{'answer': '1971', 'provenance': [{'wikipedia_id': '2230037', 'title': 'Kennedy half dollar', 'start_paragraph_id': 3, 'start_character': 3, 'end_paragraph_id': 3, 'end_character': 7, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '2230037', 'title': 'Kennedy half dollar', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 585, 'bleu_score': 0.980097014099621, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 15320458357251487631}, 'section': 'Section::::Abstract.'}]}]