A wall has two layers A and B, each made of different material. Both the layers have the same thickness. The thermal conductivity of the material of A is twice that of B . Under thermal equilibrium, the temperature difference across the wall is `36^@C.` The temperature difference across the layer A is
A. `6^(@)`C
B. `12^(@)`C
C. `24^(@)`C
D. `18^(@)`C