This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Radiology
[{'content': 'Minimal dentinal destruction to get evident on the radiograph: \nSelect and explain. \nA. 30 microns, B. 40 microns, C. 20 microns, D. 200 microns', 'role': 'user'}, {'content': "Answer: B \nExplanation: Minimal dentinal destruction of 40 microns is required to be seen on a radiograph.\nLesions confined to the enamel may not be evident until approximately 30% to 40% demineralization has occurred.\xa0For this reason, the actual depth of penetration of a carious lesion is often deeper than seen in the image.\n\xa0\nReference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 288", 'role': 'assistant'}]