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
Preservation of cusps and marginal ridges is: 
Select and explain. 
A. Primary resistance form., B. Primary retention form., C. Convenience form., D. Secondary retention form.
A: Primary resistance form.
Answer: A: Primary resistance form. 
Explanation: Features of primary resistance form are:

Box shape.
Flat floor.
Preservation of cusps and marginal ridges.
Rounded internal and external line angles.