A Brass Bar Twisted by Torques T Acting at the Ends

A brass bar twisted by torques T acting at the ends has the follow- ing properties: L = 2.1 m, d = 38 mm, and G = 41 GPa. The torsional
Stiffness of the bar is approximately: (A) $1200 \mathrm{~N} \cdot \mathrm{m}$
(B) $2600 \mathrm{~N} \cdot \mathrm{m}$
(C) $4000 \mathrm{~N} \cdot \mathrm{m}$
(D) $4800 \mathrm{~N} \cdot \mathrm{m}$