The Iron Ball Shown Is Being Swung in a Vertical

Multiple Choice

The iron ball shown is being swung in a vertical circle at the end of a 0.7-m string. How slowly can the ball go through its top position without having the string go slack?

A) 1.3 m/s
B) 2.6 m/s
C) 3.9 m/s
D) 6.9 m/s
E) 9.8 m/s