Two solid spheres A and B made of the same material have radii $r_A$ and $r_B$ respectively. Both the sphere are cooled from the same temperature under the conditions valid for Newton's law of cooling. The ratio of the rate of change of temperature of A and B is :

Question

\[\begin {array} {1 1} (a)\;\frac{r_A}{r_B} & \quad (b)\;\frac{r_B}{r_A} \\ (c)\;\frac{r^2_A}{r^2_B} & \quad (d)\;\frac{r^2_B}{r^2_A} \end {array}\]