Indicate Which Type of T Cell Is Being Described by Writing

Indicate which type of T cell is being described by writing the appropriate letter in the blank using the following answer code:
-Serve to limit immune reactions in a check and balance relationship with the other lymphocytes.

A) applies to cytotoxic T cells
B) applies to helper T cells
C) applies to regulatory T cells
D) applies to both helper T and regulatory T cells
E) applies to all three types of T cells