Question: when did construction on the great pyramid of giza begin
[{'answer': 'c. 2560 -- 2540 BC'}, {'provenance': [{'wikipedia_id': '1979299', 'title': 'Giza pyramid complex', 'start_paragraph_id': 5, 'start_character': 0, 'end_paragraph_id': 5, 'end_character': 607, 'bleu_score': 0.929391499021567, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 14459963932590722127}, 'section': 'Section::::Pyramids and Sphinx.\n'}]}]