maximum permissible shear stress

Question

A hallow shaft ($(d_{0}=2d_{i})$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, $d_{o}$is

(a) 11.29 mm                      (b) 22.58 mm
(c) 33.87                             (d) 45.16 mm