What Is the Hydronium Ion Concentration of a 0 $\times$ 10^-5 $\times$

What is the hydronium ion concentration of a 0.20 M ammonia solution? (The K_b value for ammonia is 1.8 $\times$ 10^-5.)

A) 2.8 $\times$ 10^-10 M
B) 5.5 $\times$ 10^-10 M
C) 1.8 $\times$ 10^-5 M
D) 5.3 $\times$ 10^-12 M
E) 1.9 $\times$ 10^-3 M