Out of (ref. image), which one is more reactive towards SN2 reaction and why?

So in case of A Cl is attached to secondary carbon and in Case of B Cl is attached to primary carbon. So SN2 reaction is a reaction in which attack and removal of leaving group takes place at the same time. So, the more bulky the groups, the less the SN2 rate. As in B, chloride is attached to primary carbon, the SN2 rate is more.