John is now whirling the ball in a horizontal circle of radius r at constant speed v such that the string makes one complete revolution every second, and the tension in the string is T. John speeds up the stone, keeping the radius of the circle unchanged, so that the string now makes 2 complete revolutions every second. What happens to the tension in the string?
a. the tension reduced to half
b. the tension increases to twice
c. tension increases to 4x
d. tension reduces to 1/4
e. tension unchanged