In which of the following reactions, the equilibrium remains unaffected on addtion of small amount of argon at constant volume ?
A. `H_(2) (g) + I_(2) (g) hArr 2 HI (g)`
B. `PCl_(5)(g) hArr PCl_(3) (g) + Cl_(2) (g)`
C. `N_(2) (g) + 3 H_(2)(g) hArr 2 NH_(3) (g)`
D. The equilibrium will remain unaffected in all the three cases.