Two sheets of thickness d and 3d, are touching each other. The temperature just outside the thinner sheet is T1 and on the side of the thicker sheet is T3. The interface temperature is T2. T1, T2 and T3 are in arithmetic progression. The ratio of thermal conductivity of thinner sheet to thicker sheet is
a. 1 : 3
b. 3 : 1
c. 2 : 3
d. 3 : 9