The Secondary Signal Needed for T_H0 Helper Cells to Differentiate

The secondary signal needed for T_H0 helper cells to differentiate into either T_H1 cells or T_H2 cells is the crosslink between:

A) CD154 (on T_H0 helper cells) and CD40 (on APC cell)
B) MHC I (on T_H0 helper cells) and B7 (on APC cell)
C) CD4 (on T_H0 helper cells) and MHC II (on APC cell)
D) CD8 (on T_H0 helper cells) and MHC I (on APC cell)
E) CD28 (on T_H0 helper cells) and B7 (on APC cell)