Question: when did mcdonald's start the big mac
[{'answer': 'nationwide in 1968', 'provenance': [{'wikipedia_id': '299395', 'title': 'Big Mac', 'start_paragraph_id': 1, 'start_character': 165, 'end_paragraph_id': 1, 'end_character': 183, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'in the Greater Pittsburgh area , United States , in 1967'}, {'provenance': [{'wikipedia_id': '299395', 'title': 'Big Mac', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 229, 'bleu_score': 0.907026096796234, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 3427207191150313492}, 'section': 'Section::::Abstract.'}]}]