One mole of helium (He) gas is added to the equilibrium $N_2O_4(g) hArr 2NO_2(g)$ set up in cylinder fitted with piston in such a way that the piston is moved outwards to keep the total equilibrium pressure constant:
A. The equilibrium will *** unchanged
B. The equilibrium will shift in the forward direction
C. The equilibrium will shift in the backward direction
D. of the above
