Question: what is the most reliable method for preventing blackflow
[{'answer': 'an air gap', 'provenance': [{'wikipedia_id': '767292', 'title': 'Backflow prevention device', 'start_paragraph_id': 10, 'start_character': 77, 'end_paragraph_id': 10, 'end_character': 87, 'bleu_score': 1.0, 'section': 'Section::::Preventive devices.\n'}]}, {'provenance': [{'wikipedia_id': '767292', 'title': 'Backflow prevention device', 'start_paragraph_id': 10, 'start_character': 0, 'end_paragraph_id': 10, 'end_character': 485, 'bleu_score': 0.976327165973018, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9128743780745276278}, 'section': 'Section::::Preventive devices.\n'}]}]