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: Pathology
Which of the following is a painful ulcer? 
Select and explain. 
A. syphilitic ulcer, B. trophic ulcer, C. oral chancre, D. all the above
C: oral chancre
Answer: C: oral chancre 
Explanation: None