Question: when did the nfl start the 2 minute warning
[{'answer': "1960 's"}, {'provenance': [{'wikipedia_id': '3736373', 'title': 'Two-minute warning', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 639, 'bleu_score': 0.9526401996952971, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11432487549398706695}, 'section': 'Section::::History.\n'}]}]