What Is the Approximate Value of the Equilibrium Constant,K_n,for the Neutralization

What is the approximate value of the equilibrium constant,K_n,for the neutralization of nitrous acid with ammonia,shown in the equation below? The K_a for HNO₂ is 4.5 × 10^-4 and the K_b for NH₃ is 1.8 × 10^-5. HNO₂(aq) + NH₃(aq) ⇌ NH₄NO₂(aq)

A) 8.1 × 10⁵
B) 1.8 × 10⁹
C) 4.5 × 10¹⁰
D) 8.1 × 10¹⁹