Brass Bar Twisted by Torques T Acting at the Ends $L = 2.1 \mathrm {~m} , d = 38 \mathrm {~mm}$

brass bar twisted by torques T acting at the ends has the following properties: $L = 2.1 \mathrm {~m} , d = 38 \mathrm {~mm}$ and $G = 41 \mathrm { GPa }$ . 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) $4800 \mathrm {~N} \cdot \mathrm { m }$