In $SN_ 2$ reaction, the correct order of reactivity for the following compounds:$CH_3Cl,CH_3CH_2Cl,(CH_3)_2CHCl$ and $(CH_3)_3CCl$ is

$\begin{array}{1 1}(A)\;CH_3CH_2Cl > CH_3Cl > (CH_3)_2CHCl > (CH_3)_3CCl\\(B)\;(CH_3)_2CHCl > CH_3CH_2Cl > CH_3Cl > (CH_3)_3CCl\\(C)\;CH_3Cl > (CH_3)_2CHCl > CH_3CH_2Cl > (CH_3)_3CCl\\(D)\;CH_3Cl > CH_3CH_2Cl > (CH_3)_2CHCl > (CH_3)_3CCl\end{array}$

$CH_3Cl > CH_3CH_2Cl > (CH_3)_2CHCl > (CH_3)_3CCl$
Hence (D) is the correct answer.