There rods of the same dimensions have thermal conductivities \( 3 \mathrm{~K}, 2 \mathrm{~K} \)...

There rods of the same dimensions have thermal conductivities \( 3 \mathrm{~K}, 2 \mathrm{~K} \) and \( \mathrm{K} \). They are arranged as shown in fig. with their ends at \( 100^{\circ} \mathrm{C}, 50^{\circ} \mathrm{C} \) and \( 20^{\circ} \mathrm{C} \). The temperature of their junction is
(a) \( 60^{\circ} \)
(b) \( 70^{\circ} \)
(c) \( 50^{\circ} \)
(d) \( 35^{\circ} \)