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: Microbiology
Which of the following has only 1 serotype - 
Select and explain. 
A. C psittaci, B. C pneumoniae, C. C trachomatis, D. None
B: C pneumoniae
Answer: B: C pneumoniae 
Explanation: None