4. A child plays a string of length = 20 cm connected with a ball of mass m = 1 kg, and starts to rotate it vertically. Find the minimum speed of the ball at the top that is needed to rotate it vertically in circular fashion? (Gravitational acceleration g = 10 m/s2 ) (A) 0.7 m/s (B) 1.4 m/s (C) 2.2 m/s (D) 3.3 m/s (E) 4.2 m/s