Question: when did the roman empire reach its greatest extent
[{'answer': 'under Trajan', 'provenance': [{'wikipedia_id': '25507', 'title': 'Roman Empire', 'start_paragraph_id': 20, 'start_character': 39, 'end_paragraph_id': 20, 'end_character': 51, 'bleu_score': 1.0, 'section': 'Section::::Geography and demography.\n'}]}]