There are two long co-axial solenoids of same length `l`. The inner & outer coils have radii `r_1 & r_2` and number of turns per unit length `n_1 & n_2`, respectively. The ratio of mutual inductance to the self-inductance of the inner coil

Question

There are two long co-axial solenoids of same length `l`. The inner & outer coils have radii `r_1 & r_2` and number of turns per unit length `n_1 & n_2`, respectively. The ratio of mutual inductance to the self-inductance of the inner coil is

Options:
(a) `n_/n_1.r_1/r_2` (b) `n_2/n_1^2.r_2^2/n_1` (c) `n_2/n_1` (d) `n_1/n_2`