Question: a necker cube is an example of a(n)
[{'answer': 'an optical illusion', 'provenance': [{'wikipedia_id': '56112', 'title': 'Necker cube', 'start_paragraph_id': 1, 'start_character': 19, 'end_paragraph_id': 1, 'end_character': 38, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '56112', 'title': 'Necker cube', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 309, 'bleu_score': 0.824765868059899, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 12836386154349889772}, 'section': 'Section::::Abstract.'}]}]