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
Retention grooves for class 3 restoration are placed: 
Select and explain. 
A. Bucco axial and linguo axial line angles., B. Occlusoaxial line angles., C. Axiogingival line angle., D. Retention grooves are not possible in class 3 preparation.
C: Axiogingival line angle.
Answer: C: Axiogingival line angle. 
Explanation: None