Question: what commodity ultimately led to british gaining control of hong kong in the treaty of nanjing
[{'answer': 'opium', 'provenance': [{'wikipedia_id': '143829', 'title': 'First Opium War', 'start_paragraph_id': 2, 'start_character': 392, 'end_paragraph_id': 2, 'end_character': 397, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '143829', 'title': 'First Opium War', 'start_paragraph_id': 125, 'start_character': 0, 'end_paragraph_id': 125, 'end_character': 1528, 'bleu_score': 0.991578759953019, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8899656048393634164}, 'section': 'Section::::Legacy.\n'}]}]