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
In class 3 and class 5 cavity  preparation  retention grooves are given: 
Select and explain. 
A. Over dentinal wall., B. Over facial and lingual walls., C. Over axial wall, D. All of the above
A: Over dentinal wall.
Answer: A: Over dentinal wall. 
Explanation: In class III class IV preparations the external walls diverge outwards to provide string enamel margins and therefore Retention Coves (on 3-point angles) or Retention grooves (on axio gingival line angle) are prepared in the dentinal walls to provide the retention form.