Question: when did they start building the cn tower
[{'answer': '1973', 'provenance': [{'wikipedia_id': '6112', 'title': 'CN Tower', 'start_paragraph_id': 13, 'start_character': 50, 'end_paragraph_id': 13, 'end_character': 54, 'bleu_score': 1.0, 'section': 'Section::::History.:Construction.\n'}]}]