Excess solid NH₄I on rapid heating in a closed vessel develops a constant pressure of 4Pₐ wing to the partial decomposition of solid into NH₃(g) and HI(g) but the pressure gradually increases further owing to the dissociation of HI. The final pressure at equilibrium is 5P.Kp for 2HI(g) → H₂(g) + I₂(g) is ______.
A. 9/32
B. 81/1024
C. 1/16
D. None of these.