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
Minimal dentinal destruction to get evident on the radiograph: 
Select and explain. 
A. 30 microns, B. 40 microns, C. 20 microns, D. 200 microns
B: 40 microns
Answer: B: 40 microns 
Explanation: Minimal dentinal destruction of 40 microns is required to be seen on a radiograph.
Lesions confined to the enamel may not be evident until approximately 30% to 40% demineralization has occurred. For this reason, the actual depth of penetration of a carious lesion is often deeper than seen in the image.
 
Reference: White and Pharoah's Oral Radiology Principles and Interpretation 7th edition page no 288