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. $\dfrac{\mu_0\pi r_1^2}{2r_2}$