Question: who plays agent callen on ncis los angeles
[{'answer': "Chris O'Donnell", 'provenance': [{'wikipedia_id': '28976713', 'title': 'G. Callen', 'start_paragraph_id': 1, 'start_character': 102, 'end_paragraph_id': 1, 'end_character': 117, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '28976713', 'title': 'G. Callen', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 309, 'bleu_score': 0.826287389677645, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 7699806828892198776}, 'section': 'Section::::Abstract.'}]}]