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
Passive retention in removable partial dentures is provided by: 
Select and explain. 
A. Direct retainers, B. Indirect retainers, C. Undercuts, D. Guiding planes
D: Guiding planes
Answer: D: Guiding planes 
Explanation: None