A long solenoid of length l, cross-section A having N₁ turns has wound about its centre a small coil of N₂ turns as shown in figure Then the mutual inductance of two circuits is

• A

  $\frac{{\mathrm{\mu }}_0\mathrm{A}\left({\mathrm{N}}_1/{\mathrm{N}}_2\right)}{l}$

• B

  $\frac{{\mathrm{\mu }}_0\mathrm{A}\left({\mathrm{N}}_1{\mathrm{N}}_2\right)}{l}$

• C

  ${\mathrm{\mu }}_0{\mathrm{AN}}_1{\mathrm{N}}_2\mathrm{l}$

• D

  $\frac{{\mathrm{\mu }}_0{\mathrm{AN}}_1^2{\mathrm{N}}_2}{l}$