Question: the walls of the alveoli are mostly made of
[{'answer': 'collagen and elastic fibres'}, {'provenance': [{'wikipedia_id': '102359', 'title': 'Pulmonary alveolus', 'start_paragraph_id': 6, 'start_character': 0, 'end_paragraph_id': 6, 'end_character': 444, 'bleu_score': 0.6558659615238991, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17046099410396269149}, 'section': 'Section::::Structure.:Microanatomy.\n'}]}]