$A$ bar magnet has a magnetic moment equal to $5 \times 10^{-5} \, \text{Wb} \cdot \text{m}$. It is suspended in a magnetic field which has a magnetic induction $B$ equal to $8\pi \times 10^{-4} \, \text{T}$. The magnet vibrates with a period of vibration equal to $15 \, \text{s}$. The moment of inertia of the magnet is:

• A
  $22.5 \, \text{kg} \cdot \text{m}^2$
• B
  $11.25 \, \text{kg} \cdot \text{m}^2$
• C
  $5.62 \, \text{kg} \cdot \text{m}^2$
• D
  $7.16 \times 10^{-7} \, \text{kg} \cdot \text{m}^2$