Question: what part of an ar 15 has the serial number
[{'answer': 'lower assembly', 'provenance': [{'wikipedia_id': '491852', 'title': 'Receiver (firearms)', 'start_paragraph_id': 5, 'start_character': 694, 'end_paragraph_id': 5, 'end_character': 708, 'bleu_score': 1.0, 'section': 'Section::::In US law.\n'}]}, {'provenance': [{'wikipedia_id': '491852', 'title': 'Receiver (firearms)', 'start_paragraph_id': 5, 'start_character': 0, 'end_paragraph_id': 5, 'end_character': 847, 'bleu_score': 0.937019127799618, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 10207160850706434856}, 'section': 'Section::::In US law.\n'}]}]