Question: when did the united nations ban chemical weapons
[{'answer': '1968', 'provenance': [{'wikipedia_id': '51836', 'title': 'Chemical Weapons Convention', 'start_paragraph_id': 6, 'start_character': 90, 'end_paragraph_id': 6, 'end_character': 94, 'bleu_score': 1.0, 'section': 'Section::::History.\n'}]}, {'provenance': [{'wikipedia_id': '51836', 'title': 'Chemical Weapons Convention', 'start_paragraph_id': 6, 'start_character': 0, 'end_paragraph_id': 6, 'end_character': 935, 'bleu_score': 0.9618420214052931, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 15754213340461955779}, 'section': 'Section::::History.\n'}]}]