Question: when did 'open era' began in tennis
[{'answer': '1968', 'provenance': [{'wikipedia_id': '9719652', 'title': 'History of tennis', 'start_paragraph_id': 5, 'start_character': 3, 'end_paragraph_id': 5, 'end_character': 7, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '9719652', 'title': 'History of tennis', 'start_paragraph_id': 5, 'start_character': 0, 'end_paragraph_id': 5, 'end_character': 610, 'bleu_score': 0.96313215334665, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17033718337159795580}, 'section': 'Section::::Abstract.'}]}]