Question: which season experiences the largest percentage of tornadoes on average in the united states
[{'answer': 'spring', 'provenance': [{'wikipedia_id': '18436515', 'title': 'Tornadoes in the United States', 'start_paragraph_id': 3, 'start_character': 112, 'end_paragraph_id': 3, 'end_character': 118, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '18436515', 'title': 'Tornadoes in the United States', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 565, 'bleu_score': 0.914178283640501, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9358383094445671443}, 'section': 'Section::::Abstract.'}]}]