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
Which of the following statements about an amalgam tooth/cavity preparation is true? 
Select and explain. 
A. 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.
B: The cavosurface margin should provide for a 90- degree amalgam margin.
Answer: B: The cavosurface margin should provide for a 90- degree amalgam margin. 
Explanation: 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.