What Is the Equilibrium Concentration of Amide Ion (NH₂^-)in Liquid

Multiple Choice

What is the equilibrium concentration of amide ion (NH₂^-) in liquid ammonia at 25°C? ("am" = dissolved in ammonia)
2NH₃(l)
NH₄⁺(am) + NH₂^-(am) ; K_c = 1.8 × 10^-24 at 25°C

A) 2.6 × 10^-12 M
B) 3.6 × 10^-24 M
C) 1.3 × 10^-12 M
D) 1.8 × 10^-24 M
E) 9.0 × 10^-25 M