Question: when does thanos first appear in the mcu
[{'answer': 'The Avengers ( 2012 )'}, {'provenance': [{'wikipedia_id': '701741', 'title': 'Thanos', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 350, 'bleu_score': 0.564595243406664, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 18240521959299613399}, 'section': 'Section::::Abstract.'}]}]