The heat is flowing through two cylinderical rods of same material. The diameters of the rods are in the ratio `1:2` and their lengths are in the ratio `2:1`. If the temperature difference between their ends is the same, the ratio of rates of flow of heat through them will be
A. `1 : 1`
B. `2 : 1`
C. `1 : 4`
D. `1 : 8`