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: Dental
[{'content': 'Which of the following statements about an amalgam tooth/cavity preparation is true? \nSelect and explain. \nA. The enamel cavosurface margin angle must be 90 degrees., B. The cavosurface margin should provide for a 90- degree amalgam margin., C. All prepared walls should converge externally., D. Retention form for Class Vs can be placed at the DEJ.', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Although the amalgam margin must be 90 degrees, the enamel margin may not be 90 degrees, especially on the occlusal surface. Most walls converge occlusally, but many Class V amalgam preparations have walls that diverge externally. No retention form should be placed at the DEJ; otherwise, the adjacent enamel will be undermined and subject to fracture.', 'role': 'assistant'}]