Answer the following question given this paragraph: 

After an infection has been brought under control, most cytotoxic T cells die off. However, a few remain as memory cells. If the same pathogen enters the body again, the memory cells mount a rapid immune response. They quickly produce many copies of cytotoxic T cells specific to the antigen of that pathogen.


Q: After an infection has been brought under control and most cytotoxic t cells die off, a few remain as what?

 Choices:

- b cells

- natural killer cells

- control cells

- memory cells

A:
memory cells
