Question: who is the actress that plays alison in pretty little liars
[{'answer': 'Sasha Pieterse', 'provenance': [{'wikipedia_id': '2211870', 'title': 'Sasha Pieterse', 'start_paragraph_id': 0, 'start_character': 0, 'end_paragraph_id': 0, 'end_character': 14, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '2211870', 'title': 'Sasha Pieterse', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 457, 'bleu_score': 0.6750328915601621, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 1634011133818539745}, 'section': 'Section::::Abstract.'}]}]