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
[{'content': "Down's cephalometric analysis uses ____line as a reference plane \nSelect and explain. \nA. ANS-PN5, B. SN plane, C. Porion - Orbitale, D. Gonion- Menton", 'role': 'user'}, {'content': 'Answer: C \nExplanation: None', 'role': 'assistant'}]