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
After how many days of radiotherapy, mucositis appear: 
Select and explain. 
A. 7 days., B. 10 days., C. 14 days., D. 30 days.
C: 14 days.
Answer: C: 14 days. 
Explanation: After 12-17 days of radiotherapy, mucositis appears.