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
'Crippling Fluorosis' is characterized by: 
Select and explain. 
A. Blockage of blood vessels, B. Chipping of teeth, C. Rigid spine, D. Calcification of blood vessels
C: Rigid spine
Answer: C: Rigid spine 
Explanation: None