Question: what material is captain america's shield made out of
[{'answer': 'vibranium', 'provenance': [{'wikipedia_id': '3688898', 'title': "Captain America's shield", 'start_paragraph_id': 4, 'start_character': 285, 'end_paragraph_id': 4, 'end_character': 294, 'bleu_score': 1.0, 'section': 'Section::::Abilities.:Vibranium-steel alloy.\n'}]}, {'provenance': [{'wikipedia_id': '3688898', 'title': "Captain America's shield", 'start_paragraph_id': 4, 'start_character': 0, 'end_paragraph_id': 4, 'end_character': 794, 'bleu_score': 0.5057034677565, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5193627381741589719}, 'section': 'Section::::Abilities.:Vibranium-steel alloy.\n'}]}]