Suppose your computer's IP address is $155\mathrm{.}61\mathrm{.}9\mathrm{.}188,$ and your network administrator has not sub$-$netted this Classful Network. What is your computer's subnet mask?

a$\mathrm{.}255\mathrm{.}255\mathrm{.}255\mathrm{.}255$

b$\mathrm{.}255\mathrm{.}255\mathrm{.}0\mathrm{.}0$

c$\mathrm{.}0\mathrm{.}0\mathrm{.}0\mathrm{.}0$

d$\mathrm{.}255\mathrm{.}0\mathrm{.}0\mathrm{.}0$

e$\mathrm{.}255\mathrm{.}255\mathrm{.}255\mathrm{.}0$