Question: when did puerto rico become a us territory
[{'answer': '1950', 'provenance': [{'wikipedia_id': '23041', 'title': 'Puerto Rico', 'start_paragraph_id': 4, 'start_character': 317, 'end_paragraph_id': 4, 'end_character': 321, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]