If the $MTBF=23\mathrm{.}58$ hours, the system reliability for a mission time of $29$ hours is:

$*$a$)29\mathrm{.}23\%.$

b$)39\mathrm{.}13\%.$

c$)49\mathrm{.}83\%.$

d$)$ All alternatives are right.

e$)$ All alternatives are false.