Three rods of identical cross-section and lengths are made of three different materials of thermal conductivity $K_{1}, K_{2},$ and $K_{3}$,respectively. They are joined together at their ends to make a long rod. One end of the long rod is maintained at $100^{\circ} C$ and the other at $0^{\circ} C$. If the joints of the rod are at $70^{\circ} C$ and $20^{\circ} C$ in steady state and there is no loss of energy from the surface of the rod,the correct relationship between $K_{1}, K_{2}$ and $K_{3}$ is:

• A
  $K_{1}: K_{3}=2: 3 ; K_{2}: K_{3}=2: 5$
• B
  $K_{1}< K_{2}< K_{3}$
• C
  $K_{1}: K_{2}=5: 2 ; K_{1}: K_{3}=3: 5$
• D
  $K_{1}> K_{2}> K_{3}$