A circular loop of radius r is placed inside another circular loop of radius R (R >> r). The loops are coplanar and concentric. The mutual inductance (M) of the system is proportional to

A

$$\frac{r}{R}$$

B

$$\frac{r^2}{R}$$

C

$$\frac{R}{r}$$

D

$$\frac{r^2}{R^2}$$