Question: when did mcdonald's start selling shamrock shakes
[{'answer': '1970', 'provenance': [{'wikipedia_id': '5546576', 'title': 'Shamrock Shake', 'start_paragraph_id': 3, 'start_character': 43, 'end_paragraph_id': 3, 'end_character': 47, 'bleu_score': 1.0, 'section': 'Section::::Origin.\n'}]}, {'provenance': [{'wikipedia_id': '5546576', 'title': 'Shamrock Shake', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 328, 'bleu_score': 0.962600167205617, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 1367021978503637309}, 'section': 'Section::::Origin.\n'}]}]