The magnetic field $(B)$ inside a long solenoid having $n$ turns per unit length and carrying current $I$ when an iron core is kept in it is ($\mu_0 = $ permeability of vacuum,$\chi = $ magnetic susceptibility).

  • A
    $\mu_0 n I (1 - \chi)$
  • B
    $\mu_0 n I \chi$
  • C
    $\mu_0 n I^2 (1 + \chi)$
  • D
    $\mu_0 n I (1 + \chi)$

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