A computer has an MTTF $\backslash (=34\backslash$mathrm$\{$hr$\}\backslash )$ and an MTTR $\backslash (=2\mathrm{.}5\backslash$mathrm$\{$hr$\}\backslash ).$

If the MTTR is reduced to $1\mathrm{.}5$ hr $,$ what MTTF can be tolerated without decreasing the availability of the computer?a$.23\mathrm{.}25$b$.22$C$.20\mathrm{.}4$d$.21\mathrm{.}35$