Lead Poisoning Used to Be Treated by Injections of Ethylenediaminetetraacetic $\times$

Lead poisoning used to be treated by injections of ethylenediaminetetraacetic acid, EDTA. If the concentration of EDTA in the blood of a patient is 3.0 $\times$ 10^-9 M and the formation constant for Pb(EDTA) ^2- is 2.0 $\times$ 10¹⁸, what is the concentration ratio of free toxic Pb^2- to lead-EDTA complex in the blood?

A) 2.0 $\times$ 10^-11
B) 1.7 $\times$ 10^-10
C) 2.0 $\times$ 10^-9
D) 5.0 $\times$ 10^-19
E) 3.0 $\times$ 10^-9