If the Base Dissociation Constant of a Butyrate Ion at 25^oC

If the base dissociation constant of a butyrate ion at 25^oC is $2.20 \times 10 ^ { - 9 }$ , what is its pK_b?

A) 12.5
B) 11.6
C) 10.0
D) 7.44
E) 8.66