If the moment of inertia of a section about its axis is I and its effective sectional area is A, its radius of gyration r about the axis is

A

$$r = \frac{I}{A}$$

B

$$r = \frac{\sqrt I}{A}$$

C

$$r = \frac{\sqrt[3]{I}}{A}$$

D

$$r = \frac{\sqrt A}{I}$$