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. \(\left[ML^{2}T^{-2}A^{-2}\right]\)

Two conducting circular loops of radii \(R_1\) and \(R_2\) are placed in the same plane with their centres coinciding. If \(R_1>>R_2\), the mutual inductance \(M\) between them will be directly proportional to: