There are two long co-axial solenoids of same length l. The inner and other coils have redii `r_(1)` and `r_(2)` and number of turns per unti length `n_(1)` and `n_(2)` respectively .The reatio of mutual inductance to the self - inductance of the inner- coil is :

Options

A. `n_(1)/n_(2)`
B. `n_(2)/n_(1).r_(1)/r_(2)`
C. `n_(2)/n_1.r_2^2/r_1^2`
D. `n_(2)/n_(1)`