A Solution of NaF Is Added Dropwise to a Solution $\mathrm { Ba } ^ { 2 + }$

A solution of NaF is added dropwise to a solution that is 0.0188 mol L^-1 in $\mathrm { Ba } ^ { 2 + }$ .When the concentration of $\mathrm { F } ^ { - }$ exceeds ________ mol L^-1, $\mathrm { BaF } _ { 2 }$ will precipitate.Neglect volume changes.For $\mathrm { BaF } _ { 2 }$ , $K _ { \mathrm { sp } } = 1.7 \times 10 ^ { - 6 }$

A) 9.0 × 10^-5
B) 3.2 × 10^-8
C) 4.5 × 10^-5
D) 2.4 × 10^-3
E) 9.5 × 10^-3