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
Innate immunity active against viral cells - 
Select and explain. 
A. NK cells, B. Cytotoxic T cells, C. B cell, D. Memory B cell
A: NK cells
Answer: A: NK cells 
Explanation: NK cells are part of innate immunity
NK cells are considered to be important in immune surveillance and natural defense against virus-infected and malignant mutant cells.