A bar magnet has a magnetic moment equal to $5 \times 10^{-5} \mathrm{~Wb}-\mathrm{m} .$ It is suspended in a magnetic field which has a magnetic induetion $B$ equal to $8 \pi \times 10^{-4} \mathrm{~T}$. The magnet vibrates with a period of vibration equal to 15 s. The moment of inertia of magnet is
(a) $4.54 \times 10^{4} \mathrm{~kg}-\mathrm{m}^{2}$
(b) $4.54 \times 10^{-5} \mathrm{~kg}-\mathrm{m}^{2}$
(c) $4.54 \times 10^{-4} \mathrm{~kg}-\mathrm{m}^{2}$
(d) $4.54 \times 10^{5} \mathrm{~kg}-\mathrm{m}^{2}$