A flywheel connected to a punching machine has to supply energy of $400 Nm$ while running at a mean angular speed of $20 rad/s$. If the total fluctuation of speed is not to exceed $±2%$, the mass moment of inertia of the flywheel in $kg$-$m^2$ is

1. $25$
2. $50$
3. $100$
4. $125$