A furnace wall made of steel plate 10 mmthick and thermal conductivity 15 keal/m-hr-$$^\circ C$$ is lined inside with silica brick of 150 mm thick of thermal conductivity 1.75 kcal/m-hr-$$^\circ C$$ and on with outside magnesia bricks of 200 mm thick with thermal conductivity of 4.5 kcal/m-hr-$$^\circ C$$. The total resistance of the composite wall will be

A

$$0.13 ^\circ C-hr/kcal$$

B

$$0.23 ^\circ C-hr/kcal$$

C

$$0.03 ^\circ C-hr/kcal$$

D

$$0.33 ^\circ C-hr/kcal$$