Which S_N2 Reaction Will Occur Most Rapidly in a Mixture $\to$

Which S_N2 reaction will occur most rapidly in a mixture of water and ethanol?

A) I^- + CH₃CH₂-Br $\to$ CH₃CH₂-I + Br^-
B) I^- + CH₃CH₂-Cl $\to$ CH₃CH₂-I + Cl^-
C) I^- + CH₃CH₂-F $\to$ CH₃CH₂-I + F^-
D) Br^- + CH₃CH₂-Cl $\to$ CH₃CH₂-Br + Cl^-
E) Br^- + CH₃CH₂-F $\to$ CH₃CH₂-Br + F^-