Question: when did thriller became the best selling album
[{'answer': '1984', 'provenance': [{'wikipedia_id': '586836', 'title': 'Thriller (Michael Jackson album)', 'start_paragraph_id': 2, 'start_character': 397, 'end_paragraph_id': 2, 'end_character': 401, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '586836', 'title': 'Thriller (Michael Jackson album)', 'start_paragraph_id': 31, 'start_character': 0, 'end_paragraph_id': 31, 'end_character': 529, 'bleu_score': 0.943372970740017, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4373285206702922205}, 'section': 'Section::::Critical reception.\n'}]}]