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 of a proximal box in a class II cavity prepared for amalgam should be: \nSelect and explain. \nA. Sharp and elongated at the dentino enamel junction of the buccal and lingual walls, B. Sharp and well defined extending from the gingival to the occlusal cavosurface angle along the buccal and lingual line angles, C. Elongated and rounded in dentine at the buccoaxial and linguoaxial Line angles extending from the gingival wall to the axiopulpal line angle, D. Short and rounded at the dentino enamel junction of the buccal and lingual walls', 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]