Picric Acid Has Been Used in the Leather Industry and in Etching

Picric acid has been used in the leather industry and in etching copper. However, its laboratory use has been restricted because it dehydrates on standing and can become shock sensitive. It has an acid dissociation constant of 0.42. What is the [H₃O⁺] for a 0.20 M solution of picric acid?

A) 0.022 M
B) 0.052 M
C) 0.15 M
D) 0.20 M
E) 0.29 M