A hollow shaft ($d_o$ = $2d_i$ where $d_o$ and $d_i$ are the outer and inner diameters respectively) needs to transmit $20$ $kW$ power at $3000$ $RPM$. If the maximum permissible shear stress is $30$ $MPa$, do is
- $11.29$ $mm$
- $22.58$ $mm$
- $33.87$ $mm$
- $45.16$ $mm$