At a temperature ' $t$ ' the moment of inertia of a body is $I$. When the temperature of the body is increased from $t+\Delta t$, its moment of inertia also increases from $I$ to $(I+\Delta l) .$ If coefficient of linear expansion of the body is $\alpha$ ', then the ratio $\frac{\Delta I}{I}$ is :
(a) $\frac{\Delta t}{t}$
(b) $\frac{2 \Delta t}{t}$
(c) $\alpha \Delta t$
(d) $2 \alpha \Delta t$