Equilibrium constant for the reaction, NH₄OH + H₄⁺ ⇌ NH₄⁺ + H₂O is 1.8 × 10⁹. Hence, equilibrium constant for NH3(aq) + H₂O ⇌ NH₄⁺ + OH¯ is:

A. 1.8 x 10⁻⁵
B. 1.8 x 10⁵
C. 5.59 × 10⁻¹⁰
D. 1.8 x 10 ⁻⁹