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': 'The structure marked with arrow is most likely: \nSelect and explain. \nA. Pulp obliteration, B. Reactionary dentin, C. Calculus, D. Plaque', 'role': 'user'}, {'content': 'Answer: C \nExplanation: Because of its relatively low radiodensity, calculus is better visualized on images made with reduced exposure.', 'role': 'assistant'}]