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': 'Retentive grooves in the proximal box of a class II amalgam cavity should be \nSelect and explain. \nA. Sharp and elongated at the dentinoenamel junction, B. Round and elongated in the dentin, C. Round and short in the dentin, D. Sharp and elongated in the dentin', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]