A short solenoid (length $l$ and radius $r$ with $n$ turns per unit length) lies well inside and on the axis of a very long, coaxial solenoid (length $L$, radius $R$ and $N$ turns per unit length, with $R>r$ ). Current $I$ follows in the short solenoid. Choose the correct statement.

  • [KVPY 2019]
  • A

    There is uniform magnetic field $\mu_{0} n I$ in the long solenoid.

  • B

    Mutual inductance of the solenoids is $\pi \mu_{ 0 } r^{2} n N l$.

  • C

    Flux through outer solenoid due to current $I$ in the inner solenoid is proportional to the ratio $R / r$.

  • D

    Mutual inductance of the solenoids is $\pi \mu_{0} r R n N l L /(r R)^{1 / 2} .$

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