The equilibrium constant for the reaction $CaSO_4.5H_2O (s) \rightleftharpoons CaSO_4.3H_2O (s) + 2H_2O (g)$ is equal to

Question

(a) $ \frac {[CaSO_4.3H_2O] [H_2O]^2} {[CaSO_4.5H_2O]} $

(b) $ \frac {[CaSO_4.3H_2O]} {[CaSO_4.5H_2O]}$

(c) $ [H_2O]^2 $

(d) $ [H_2O] $