Question: when did the united nations designate henderson island a world heritage site
[{'answer': '1988', 'provenance': [{'wikipedia_id': '932136', 'title': 'Henderson Island (Pitcairn Islands)', 'start_paragraph_id': 3, 'start_character': 175, 'end_paragraph_id': 3, 'end_character': 179, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '932136', 'title': 'Henderson Island (Pitcairn Islands)', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 179, 'bleu_score': 0.934334813046729, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 15495146067085619374}, 'section': 'Section::::Abstract.'}]}]