How does the immune system initiate responses against intracellular pathogens like viruses, when they are hidden inside cells?

A)The immune system attacks intracellular pathogens primarily by killing cells that have already been infected.
B)The immune system only attacks these intracellular pathogens when they are found in the extracellular space.
C)Cells of the immune system secrete proteins that penetrate infected cells and attack the pathogens intracellularly.
D)All of these are correct.