One end of a thermally insulated rod is kept at a temperature $T_1$ and the other at $T_2$. The rod is composed of two section of length $l_1$ and $l_2$ and thermal conductivities $K_1$ and $K_2$ respectively. The temperature at the interface of the two sections is <br>

Question

( A ) $(K_1l_1T_1 + K_2l_2T_2) / (K_1l_1 + K_2l_2)$
( B ) $(K_2l_1T_1 + K_1l_2T_2) / (K_2l_1 + K_1l_2)$
( C ) $(K_1l_2T_1 + K_2l_1T_2) / (K_1l_2 + K_2l_1)$
( D ) $(K_2l_2T_1 + K_1l_1T_2) / (K_1l_1 + K_2l_2)$