Question: when did air pollution start in mexico city
[{'answer': '1950s', 'provenance': [{'wikipedia_id': '27358255', 'title': 'Air pollution in Mexico City', 'start_paragraph_id': 3, 'start_character': 173, 'end_paragraph_id': 3, 'end_character': 178, 'bleu_score': 1.0, 'section': 'Section::::History.\n'}]}, {'provenance': [{'wikipedia_id': '27358255', 'title': 'Air pollution in Mexico City', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 221, 'bleu_score': 0.9517713210466091, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 14940802782169630067}, 'section': 'Section::::History.\n'}]}]