What Is K_b1 for the Citrate Ion, Given That the K_a1

What is K_b1 for the citrate ion, given that the K_a1 for citric acid is 8.40 $\times$ 10^-4?

A) 2.40 $\times$ 10^-12
B) 6.23 $\times$ 10^-8
C) 6.17 $\times$ 10^-10
D) 4.80 $\times$ 10^-11
E) 1.19 $\times$ 10^-11