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 $0\mathrm{.}84,$ what

is the least reliable component B that can be used if component A has a reliability of $0\mathrm{.}75?$

a$.0\mathrm{.}893$

b$.0\mathrm{.}520$

c$.0\mathrm{.}678$

d$.0\mathrm{.}960$