What Is the Concentration of [OH^-] in a 0 $\times$ 10^-5 $\times$

What is the concentration of [OH^-] in a 0.20 M solution of ammonia? The K_b value for ammonia is 1.8 $\times$ 10^-5.

A) 3.6 $\times$ 10^-6 M
B) 1.8 $\times$ 10^-5 M
C) 0.20 M
D) 1.9 $\times$ 10^-3 M
E) 4.2 $\times$ 10^-4 M