Question: the crystal structure of metal is generally examined by
[{'answer': 'optical microscopes', 'provenance': [{'wikipedia_id': '2189901', 'title': 'Microstructure', 'start_paragraph_id': 1, 'start_character': 574, 'end_paragraph_id': 1, 'end_character': 593, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '2189901', 'title': 'Microstructure', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 1336, 'bleu_score': 0.9605610465328661, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 3884371538438223234}, 'section': 'Section::::Abstract.'}]}]