The moment of inertia of a thin rod about an axis passing through its mid point and perpendicular to the rod is $2400{\text{g cm}}^2$. The length of the $400\text{g}$ rod is nearly:

1. A
   $8.5\text{cm}$
2. B
   $17.5\text{cm}$
3. C
   $20.7\text{cm}$
4. D
   $72.0\text{cm}$