The outer faces of a rectangular slab made of equal thickness of iron and brass are maintained at `100^(@)C` and `0^(@)C` respectively. The temperature at the interface is (Thermal conductivity of iron and brass are 0.2 and 0.3 respectively)
A. `100^(@)C`
B. `40^(@)C`
C. `50^(@)C`
D. `70^(@)C`