In a $S_N2$ substitution reaction of the type $R-Br+Cl^-\underrightarrow{DMF} R-Cl+Br^-$ Which one of the following has the highest relative rate ?

Question

( A )
( B )
( C ) $CH_3– C – CH_2 Br$
( D ) $CH_3–CH_2 –CH_2 Br$