Question: when did starcraft 2 become free to play
[{'answer': 'During BlizzCon 2017', 'provenance': [{'wikipedia_id': '18717529', 'title': 'StarCraft II: Wings of Liberty', 'start_paragraph_id': 4, 'start_character': 0, 'end_paragraph_id': 4, 'end_character': 20, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '18717529', 'title': 'StarCraft II: Wings of Liberty', 'start_paragraph_id': 4, 'start_character': 0, 'end_paragraph_id': 4, 'end_character': 429, 'bleu_score': 0.8171462963198141, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4896363247248664746}, 'section': 'Section::::Abstract.'}]}]