What Is the Actual Concentration of Molecular NH₃ in a 0.200

What is the actual concentration of molecular NH₃ in a 0.200 M solution of ammonia? The K_b value for ammonia is 1.80 $\times$ 10^-5.

A) 0.200 M
B) 0.198 M
C) 1.80 $\times$ 10^-5 M
D) 1.90 $\times$ 10^-3 M
E) 3.6 $\times$ 10^-6 M