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
Primary teeth have 
Select and explain. 
A. Thinner root canals than permanent teeth., B. Higher pulp horns than permanent teeth., C. Crown root ratio of 1:4, D. All of the above
D: All of the above
Answer: D: All of the above 
Explanation: None