Question: why is it important for veins to have valves
[{'answer': 'to prevent backflow', 'provenance': [{'wikipedia_id': '53227', 'title': 'Vein', 'start_paragraph_id': 2, 'start_character': 103, 'end_paragraph_id': 2, 'end_character': 122, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '53227', 'title': 'Vein', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 122, 'bleu_score': 0.910879922930628, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17766120831325393781}, 'section': 'Section::::Abstract.'}]}]