$A$ magnetic needle has a magnetic moment of $5 \times 10^{-2} \text{ A m}^2$ and a moment of inertia of $8 \times 10^{-6} \text{ kg m}^2$. It has a period of oscillation of $2 \text{ s}$ in a magnetic field $\vec{B}$. The magnitude of the magnetic field is approximately:

  • A
    $3.2 \times 10^{-4} \text{ T}$
  • B
    $1.6 \times 10^{-4} \text{ T}$
  • C
    $0.8 \times 10^{-4} \text{ T}$
  • D
    $0.4 \times 10^{-4} \text{ T}$

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