Write a expression for mutual inductance of two coaxial solenoids.

Question

(a) $ M = \frac {\mu_0 N_1 N_2 A}{Al}$
(b) $ M = \frac {\mu_0 N_1 N_2 A}{l}$
(c) $ M = \frac {\mu_0 N_1A}{N_2 l}$
(d) $ M = \frac {\mu_0 N_2 A}{N_1 l}$