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 is not required for retention of cast gold restoration? 
Select and explain. 
A. Cavosurface bevel, B. Parallel walls, C. Beveled axiopulpal line angle, D. Undermining dentinal walls
D: Undermining dentinal walls
Answer: D: Undermining dentinal walls 
Explanation: Retention form is attained by

Creating a maximum of 2-degree occlusal divergence of the vertical walls,
Accentuating some line angles, and
Extending the lingual surface groove to create an axial wall height in this extension of at least 2.5 mm occlusogingivally.

Sturdevant Operative Dentistry 7th edition page e110