Question: who is the singer for system of a down
[{'answer': 'Serj Tankian', 'provenance': [{'wikipedia_id': '149809', 'title': 'Serj Tankian', 'start_paragraph_id': 0, 'start_character': 0, 'end_paragraph_id': 0, 'end_character': 12, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '149809', 'title': 'Serj Tankian', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 297, 'bleu_score': 0.6255664411264861, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13887918565993988361}, 'section': 'Section::::Abstract.'}]}]