An engineer designs a system with two components, A and B$.$ Both A and B have a backup of identical reliability, and both A and B work in series for the entire system to operate. If the overall system reliability must be at least $0\mathrm{.}8,$ what is the least reliable component $B$ that can be used if component A has a reliability of $0\mathrm{.}7?$

a$.0\mathrm{.}775$

b$.0\mathrm{.}714$

c$.0\mathrm{.}652$

d$.0\mathrm{.}837$