Three rods of identical cross-section and lengths are made of three different materials of thermal conductivity K1, K2 and K3, respectively. They are joined together at their ends to make a long rod (see figure). One end of the long rod is maintained at 100°C and the other at 0°C (see figure). If the joints of the rod are at 70°C and 20°C in steady state and there is no loss of energy from the surface of the rod, the correct relationship between K1, K2 and K3 is
(1) K1 : K3 = 2: 3
K2 :K3 = 2:5
(2) K1 < K2 < K3
(3) K1 :K2 = 5 : 2,
K1 :K3= 3:5
(4) K1 > K2 > K3
