Question: when did the center of the western art world shift from europe to the u.s
[{'answer': 'in the 1940s', 'provenance': [{'wikipedia_id': '147847', 'title': 'Abstract expressionism', 'start_paragraph_id': 1, 'start_character': 103, 'end_paragraph_id': 1, 'end_character': 115, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '147847', 'title': 'Abstract expressionism', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 630, 'bleu_score': 0.9815648442794911, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5672938168327466161}, 'section': 'Section::::Abstract.'}]}]