Question: when did timbuktu become a world heritage site
[{'answer': 'in December 1988', 'provenance': [{'wikipedia_id': '254061', 'title': 'Timbuktu', 'start_paragraph_id': 54, 'start_character': 28, 'end_paragraph_id': 54, 'end_character': 44, 'bleu_score': 1.0, 'section': 'Section::::Arts and culture.:World Heritage Site.\n'}]}, {'provenance': [{'wikipedia_id': '254061', 'title': 'Timbuktu', 'start_paragraph_id': 54, 'start_character': 0, 'end_paragraph_id': 54, 'end_character': 211, 'bleu_score': 0.9402297675816911, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 10692617246477394787}, 'section': 'Section::::Arts and culture.:World Heritage Site.\n'}]}]