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
Down's cephalometric analysis uses ____line as a reference plane 
Select and explain. 
A. ANS-PN5, B. SN plane, C. Porion - Orbitale, D. Gonion- Menton
C: Porion - Orbitale
Answer: C: Porion - Orbitale 
Explanation: None