Question: what is the largest source of energy in the united states
[{'answer': 'fossil fuels', 'provenance': [{'wikipedia_id': '7635170', 'title': 'Energy in the United States', 'start_paragraph_id': 4, 'start_character': 31, 'end_paragraph_id': 4, 'end_character': 43, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]