Question: which area in the united states has the most serious urban smog problem
[{'answer': 'Los Angeles', 'provenance': [{'wikipedia_id': '17024928', 'title': 'Air pollution in the United States', 'start_paragraph_id': 16, 'start_character': 265, 'end_paragraph_id': 16, 'end_character': 276, 'bleu_score': 1.0, 'section': 'Section::::International pollution.\n'}]}, {'provenance': [{'wikipedia_id': '17024928', 'title': 'Air pollution in the United States', 'start_paragraph_id': 87, 'start_character': 12, 'end_paragraph_id': 87, 'end_character': 740, 'bleu_score': 0.6570082889012471, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 10867820176022927288}, 'section': 'Section::::Air pollution in California.:Los Angeles air pollution.\n'}]}]