Two concentric circular coils, one of small radius \(\mathrm{r_1}\) and the other of large radius \(\mathrm{r_2},\) such that \(\mathrm{r_1<<r_2},\)  are placed co-axially with centres coinciding. The mutual inductance of the arrangement is:

$1. \frac{{\mathrm{\mu }}_0\mathrm{\pi }{{\mathrm{r}}_1}^2}{3{\mathrm{r}}_2}$
$2. \frac{2{\mathrm{\mu }}_0\mathrm{\pi }{{\mathrm{r}}_1}^2}{{\mathrm{r}}_2}$
$3. \frac{{\mathrm{\mu }}_0\mathrm{\pi }{{\mathrm{r}}_1}^2}{{\mathrm{r}}_2}$
$4. \frac{{\mathrm{\mu }}_0\mathrm{\pi }{{\mathrm{r}}_1}^2}{2{\mathrm{r}}_2}$

The dimensions of mutual inductance \((M)\) are:
1. \([M^2LT^{-2}A^{-2}]\)
2. \([MLT^{-2}A^{2}]\)
3. \([M^{2}L^{2}T^{-2}A^{2}]\)
4. \([ML^{2}T^{-2}A^{-2}]\)