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
The classic opportunistic infection in acquired immune deficiency syndrome is: 
Select and explain. 
A. Apthous stomatitis, B. Tuberculosis, C. Pneumocystis carinii pneumonia, D. Herpetic gingivostomatitis
C: Pneumocystis carinii pneumonia
Answer: C: Pneumocystis carinii pneumonia 
Explanation: None