Select the best explanation of how a t cell uses either cell-to-cell contact or chemical signaling to enhance the activities of the immune system.
a. chemicals called cytokines are secreted from t cells to signal production of antibodies by b cells.
b. the second exposure of the antigen causes the t cell to activate memory t cells.
c. the pathogen is engulfed by a t cell and its antigenic markers are displayed on its surface.
d. the t cell stimulates the antigen-presenting cell to release chemicals called cytokines to stimulate the cytotoxic t cells.