Question: which isotope is used as relative scale to find atomic mass
[{'answer': 'carbon - 12'}, {'provenance': [{'wikipedia_id': '213968', 'title': 'Relative atomic mass', 'start_paragraph_id': 6, 'start_character': 0, 'end_paragraph_id': 6, 'end_character': 447, 'bleu_score': 0.873413441603987, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 15077860247165308232}, 'section': 'Section::::Definition.\n'}]}]