Two concentric circular coils, one of small radius \({r_1}\) and the other of large radius \({r_2},\) such that \({r_1<<r_2},\)  are placed co-axially with centres coinciding. The mutual inductance of the arrangement is:
1. \(\frac{\mu_0\pi r_1^2}{3r_2}\)
2. \(\frac{2\mu_0\pi r_1^2}{r_2}\)
3. \(\frac{\mu_0\pi r_1^2}{r_2}\)
4. \(\frac{\mu_0\pi r_1^2}{2r_2}\)

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