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
NK cells are effective against viral infected cells only if the cell with infection: 
Select and explain. 
A. Express MHC class I proteins, B. Unable to express MHC class I proteins, C. Express MHC class II proteins, D. Unable to express MHC class II proteins
B: Unable to express MHC class I proteins
Answer: B: Unable to express MHC class I proteins 
Explanation: None