Question: when did ben 10 alien force come out
[{'answer': 'April 18 , 2008', 'provenance': [{'wikipedia_id': '14028652', 'title': 'Ben 10: Alien Force', 'start_paragraph_id': 2, 'start_character': 64, 'end_paragraph_id': 2, 'end_character': 82, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '14028652', 'title': 'Ben 10: Alien Force', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 364, 'bleu_score': 0.9699161750394991, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 600064418263834971}, 'section': 'Section::::Abstract.'}]}]