Two bars of thermal conductivities `K` and `3K` and lengths `1 cm` and `2 cm` respectively have equal cross-sectional area, they are joined lengths wise as shown in the figure. If the temperature at the ends of this composite br is `0^(@)C` and `K^(2)//l` respectively (see figure), then the temperature `phi` of the interface is
A. `50^(@)`C
B. `((100)/(3))^(@)`C
C. `60^(@)`C
D. `((200)/(3))^(@)`C
