Question: what is the biggest grocery store chain in the us
[{'answer': 'Walmart', 'provenance': [{'wikipedia_id': '549347', 'title': 'List of supermarket chains in the United States', 'start_paragraph_id': 16, 'start_character': 12, 'end_paragraph_id': 16, 'end_character': 19, 'bleu_score': 1.0, 'section': 'Section::::National chains.\n'}]}]