Question: where does positive selection of t cells occur
[{'answer': 'the thymus', 'provenance': [{'wikipedia_id': '170417', 'title': 'T cell', 'start_paragraph_id': 1, 'start_character': 51, 'end_paragraph_id': 1, 'end_character': 61, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]