Question: when did the last solar eclipse happen over the united states
[{'answer': 'August 21 , 2017', 'provenance': [{'wikipedia_id': '4762233', 'title': 'Solar eclipse of August 21, 2017', 'start_paragraph_id': 1, 'start_character': 21, 'end_paragraph_id': 1, 'end_character': 37, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '4762233', 'title': 'Solar eclipse of August 21, 2017', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 518, 'bleu_score': 0.9299049081700431, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 12473794274097390320}, 'section': 'Section::::Abstract.'}]}]