Two slabs `A` and `B` of equal surface area are placed one over the other such that their surfaces are completely in contact. The thickness of slab `A` is twice that of `B`. The coefficient of thermal conductivity of slab `B` is maintained at `25^(@)C`. The temperature at the contact of their surface is
A. `62.5^(@)C`
B. `45^(@)C`
C. `55^(@)C`
D. `85^(@)C`