Given the Following Acid Dissociation Constants,
K_a (H₃PO₄)= 7 $\times$ 10^-3
K_a (NH₄⁺)= 5

Given the following acid dissociation constants,
K_a (H₃PO₄) = 7.5 $\times$ 10^-3
K_a (NH₄⁺) = 5.6 $\times$ 10^-10
Determine the equilibrium constant for the reaction below at 25 ^$\circ$ C.H₃PO₄(aq) + NH₃(aq) NH₄⁺(aq) + H₂PO₄^-(aq)

A) 4.2 $\times$ 10^-12
B) 7.5 $\times$ 10^-8
C) 4.2 $\times$ 10²
D) 1.3 $\times$ 10⁷
E) 2.4 $\times$ 10¹¹