the torque ‘T’ must be reduced by

Question

A solid shaft of diameter 100mm, length 1000 mm is subjected to a twisting moment ‘T’, the maximum shear stress developed in the shaft is 60N/mm². A hole of 50 mm diameter is now drilled throughout length of the shaft. To developed a maximum shear stress of 60N/mm² in the hollow shaft, the torque ‘T’ must be reduced by

(a) T/4           (b) T/8    (c) T/12     (d) T/16