In a `S_(N^(2))` substitution reaction of the type
`R-Br+Cl^(-)overset("DMF")rarrR-Cl+Br^(-)`
Which one of the following has the highest relative rate?
A. `(CH_(3))_(3)C-CH_(2)Br`
B. `CH_(3)CH_(2)Br`
C. `CH_(3)CH_(2)CH_(2)Br`
D. `(CH_(3))_(2)CH-CH_(2)Br`