Question: when did the construction of burj khalifa start
[{'answer': '6 January 2004', 'provenance': [{'wikipedia_id': '794957', 'title': 'Burj Khalifa', 'start_paragraph_id': 7, 'start_character': 22, 'end_paragraph_id': 7, 'end_character': 36, 'bleu_score': 1.0, 'section': 'Section::::Development.\n'}]}, {'provenance': [{'wikipedia_id': '794957', 'title': 'Burj Khalifa', 'start_paragraph_id': 7, 'start_character': 0, 'end_paragraph_id': 7, 'end_character': 520, 'bleu_score': 0.702082577362933, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 9306197475599237081}, 'section': 'Section::::Development.\n'}]}]