For caries to be imaged on a radiograph, approximately what percentage of calcium and phosphorus of the tooth structure must be lost?

a. 20–30 percent
b. 30–40 percent
c. 40–50 percent
d. 50–60 percent

c. Because the percentage of tooth structure loss must be 40–50%, the depth of caries penetration is clinically deeper than it appears on the radiograph.