Question: who does the voice of stan on south park
[{'answer': 'Trey Parker', 'provenance': [{'wikipedia_id': '9912247', 'title': 'List of South Park cast members', 'start_paragraph_id': 1, 'start_character': 61, 'end_paragraph_id': 1, 'end_character': 72, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '9912247', 'title': 'List of South Park cast members', 'start_paragraph_id': 15, 'start_character': 0, 'end_paragraph_id': 15, 'end_character': 279, 'bleu_score': 0.9560167778278961, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 7391901957932417958}, 'section': 'Section::::Current cast.:Trey Parker.\n'}]}]