Question: structure that prevents backflow of fluid in lymphatic vessels
[{'answer': 'A valve system', 'provenance': [{'wikipedia_id': '641160', 'title': 'Lymphatic vessel', 'start_paragraph_id': 8, 'start_character': 399, 'end_paragraph_id': 8, 'end_character': 413, 'bleu_score': 1.0, 'section': 'Section::::Structure.:Lymph capillaries.\n'}]}, {'provenance': [{'wikipedia_id': '641160', 'title': 'Lymphatic vessel', 'start_paragraph_id': 8, 'start_character': 0, 'end_paragraph_id': 8, 'end_character': 739, 'bleu_score': 0.8793632971083231, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17357339024782934374}, 'section': 'Section::::Structure.:Lymph capillaries.\n'}]}]