The inner and outer wall of a furnace are at a temperature of $700C$ and $100C$ respectively. The composite wall is made of two substances, $10$ and $20cm$ thick with thermal conductivities of $0\mathrm{.}05$ and $0\mathrm{.}1W.$m$-1.C-1$ respectively. The contact resistance between two layer is $0\mathrm{.}05m2\frac{K}{W}.$ The temperature drop across the resistance is

a$)7\mathrm{.}4$

b$)8\mathrm{.}5$

c$)9\mathrm{.}5$

d$)10\mathrm{.}75$