At Sufficiently High Temperatures,ammonium Iodide Decomposes to Ammonia and Hydrogen

At sufficiently high temperatures,ammonium iodide decomposes to ammonia and hydrogen iodide. NH₄I(s) ⇌ NH₃(g) + HI(g)
A mass of 5.00 g of NH₄I is sealed in a 2.00-L flask and heated to 673 K.If 2.56 g NH₄I(s) remain unreacted when the system has reached equilibrium,what is the equilibrium constant (K_p) for the reaction? (R = 0.0821 L⋅atm/mol⋅K)

A) 0.15
B) 0.22
C) 0.47
D) 0.59
E) 0.89