Steel ring of radius rand cross-section area $A$ is fitted on to a wooden disc of radius $R(R>r) .$ If Young's modulus be $E$, then the force with which the steel ring is expanded is:(a) $A E \frac{R}{r}$
(b) $A E \frac{(R-r)}{r}$
(c) $\frac{E(R-r)}{A} \frac{(R-r)}{r}$
(d) $\frac{E}{A} \frac{r}{R}$