A $0\mathrm{.}5$ m thick plane wall has its two surfaces kept at $300d$ and

$200C.$ Thermal conductivity of the wall varies linearly with

temperature and its values at $300C$ and $200C$ are $25\frac{W}{m}K$ and

$15\frac{W}{m}K$ respectively. Then the steady heat flux through the wall is:

$($a$)8k\frac{W}{m^2}$

$($b$)5k\frac{W}{m^2}$

$($c$)4k\frac{W}{m^2}$

$($d$)3k\frac{W}{m^2}$