A 20-Kg Stone Is Tied to a 0

A 2.0-kg stone is tied to a 0.50-m long string and swung around a circle at a constant angular velocity of 12 rad/s.The net torque on the stone about the center of the circle is:

A) 0 N∙m
B) 6.0 N∙m
C) 12 N∙m
D) 72 N∙m
E) 140 N∙m