Question: which words refers to any system that uses an exchange medium
[{'answer': 'currency', 'provenance': [{'wikipedia_id': '5665', 'title': 'Currency', 'start_paragraph_id': 1, 'start_character': 2, 'end_paragraph_id': 1, 'end_character': 10, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '5665', 'title': 'Currency', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 787, 'bleu_score': 0.7137785893312221, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 15570621304180027038}, 'section': 'Section::::Abstract.'}]}]