Which Alkyl Halide Is the Most Reactive in an S_N2 $\left( \mathrm { CH } _ { 3 } \right) \mathrm { CCH } _ { 2 } \mathrm { CH } _ { 2 } \mathrm {~F}$

Which alkyl halide is the most reactive in an S_N2 reaction?

A) $\left( \mathrm { CH } _ { 3 } \right) \mathrm { CCH } _ { 2 } \mathrm { CH } _ { 2 } \mathrm {~F}$
B) $\left( \mathrm { CH } _ { 3 } \right) \mathrm { CCH } _ { 2 } \mathrm { CH } _ { 2 } \mathrm { Cl }$
C) $\left( \mathrm { CH } _ { 3 } \right) \mathrm { CCH } _ { 2 } \mathrm { CH } _ { 2 } \mathrm { Br }$
D) $\left( \mathrm { CH } _ { 3 } \right) \mathrm { CCH } _ { 2 } \mathrm { CH } _ { 2 } \mathrm { I }$
E) All primary halides react at the same rate in S_N2 reactions.