Two cylindrical rods of same meterial have the same temperature difference between their ends. The ratio of rates of flow of heat through them is 1 : 8. The ratio of the radii of the rods are 1 : 2. What is the ratio of their lengths ?

A. `2 : 1`

B. `4 : 1`

C. `1 : 8`

D. `1 : 32`