Question: which unit is used to measure mass in the metric system
[{'answer': 'kilogram', 'provenance': [{'wikipedia_id': '44142', 'title': 'Metric system', 'start_paragraph_id': 2, 'start_character': 86, 'end_paragraph_id': 2, 'end_character': 94, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]