Question: when did the senate changed to 60 votes
[{'answer': '1975', 'provenance': [{'wikipedia_id': '1612372', 'title': 'Nuclear option', 'start_paragraph_id': 7, 'start_character': 226, 'end_paragraph_id': 7, 'end_character': 230, 'bleu_score': 1.0, 'section': 'Section::::Background.:The 60-vote rule.\n'}]}, {'provenance': [{'wikipedia_id': '1612372', 'title': 'Nuclear option', 'start_paragraph_id': 7, 'start_character': 0, 'end_paragraph_id': 7, 'end_character': 509, 'bleu_score': 0.9176894347072521, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17046276781062871204}, 'section': 'Section::::Background.:The 60-vote rule.\n'}]}]