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. \(\dfrac{\mu_0\pi r_1^2}{3r_2}\)

2. \(\dfrac{2\mu_0\pi r_1^2}{r_2}\)

3. \(\dfrac{\mu_0\pi r_1^2}{r_2}\)

4. \(\dfrac{\mu_0\pi r_1^2}{2r_2}\)