Question: when did the usa join the core of the modern world system
[{'answer': 'the 20th century', 'provenance': [{'wikipedia_id': '1582312', 'title': 'Core countries', 'start_paragraph_id': 2, 'start_character': 586, 'end_paragraph_id': 2, 'end_character': 602, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '1582312', 'title': 'Core countries', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 810, 'bleu_score': 0.8149161903259351, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5849606782290877908}, 'section': 'Section::::Abstract.'}]}]