A 0.50-kg mass attached to the end of a string swings in a vertical circle with a radius of 2.0 m. When the string is horizontal, the speed of the mass is 8.0 m/s. What is the magnitude of the force of the string on the mass at this position?
(a) 16 N
(b) 17 N
(c) 21 N
(d) 11 N
(e) 25 N