Question: where does most air pollution in cities come from
[{'answer': 'chemicals', 'provenance': [{'wikipedia_id': '17024928', 'title': 'Air pollution in the United States', 'start_paragraph_id': 1, 'start_character': 37, 'end_paragraph_id': 1, 'end_character': 46, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '17024928', 'title': 'Air pollution in the United States', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 952, 'bleu_score': 0.6222454384057701, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11243941731480901299}, 'section': 'Section::::Abstract.'}]}]