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

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$.21\mathrm{.}35$