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
The structure marked with arrow is most likely: 
Select and explain. 
A. Pulp obliteration, B. Reactionary dentin, C. Calculus, D. Plaque
C: Calculus
Answer: C: Calculus 
Explanation: Because of its relatively low radiodensity, calculus is better visualized on images made with reduced exposure.