Question: what physical feature forms the eastern boundary of europe
[{'answer': 'the Ural Mountains', 'provenance': [{'wikipedia_id': '1187515', 'title': 'Geography of Europe', 'start_paragraph_id': 1, 'start_character': 326, 'end_paragraph_id': 1, 'end_character': 344, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '1187515', 'title': 'Geography of Europe', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 1168, 'bleu_score': 0.941263058501921, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 2061385391922919210}, 'section': 'Section::::Abstract.'}]}]