In SN2 reactions, the correct order of reactivity for the following compounds CH3CI, CH3CH2CI, (CH3)2CHCI, (CH3)2 CCI is
a. CH3CI > (CH3)2CHCI > CH3CH2CI > (CH3)3 CCI
b. CH3CI > CH3CH2CI > (CH3)2 CHCI > (CH3)3 CCI
c. CH3CH2CI > CH3CI > (CH3)2 CHCI > (CH3)3 CCI
d. (CH3)2 CHCI > CH3CH2CI > CH3CI > (CH3)3 CCI