Two rods A and B of different materials are welded together as shown in figure. Their thermal conductivities are $K_1$ and $K_2$. The thermal conductivity of the composite rod will be <br>

Question

( A ) $\frac{3(K_1 + K_2)}{2}$
( B ) $2 (K_1 + K_2)$
( C ) $K_1 + K_2$
( D ) $\frac{K_1 + K_2}{2}$