A solid shaft of diameter ‘D’ caries a twisting moment that develops maximum shear stress $\tau$. If the shaft is replaced by hollow one of outside diameter ‘D’ and inside diameter D/2, then the maximum shear stress will be
(a) 1.067 $\tau$ (b) 1.143 $\tau$
(c) 1.333 $\tau$ (d) 2 $\tau$