Question: when did robben island became a world heritage site
[{'answer': '1999', 'provenance': [{'wikipedia_id': '26122', 'title': 'Robben Island', 'start_paragraph_id': 12, 'start_character': 185, 'end_paragraph_id': 12, 'end_character': 189, 'bleu_score': 1.0, 'section': 'Section::::History.\n'}]}]