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
By using phase contrast microscope: 
Select and explain. 
A. Internal structures are differentiated in living cells, B. Internal structures are differentiated in dead cells, C. Internal metabolic activities can be observed, D. External capsule formation can be observed
A: Internal structures are differentiated in living cells
Answer: A: Internal structures are differentiated in living cells 
Explanation: None