A Ball Is Whirled on the End of a String

A ball is whirled on the end of a string in a horizontal circle of radius R at constant speed v. Complete the following statement: The centripetal acceleration of the ball can be increased by a factor of 4 by

A) keeping the speed fixed and increasing the radius by a factor of 4.
B) keeping the radius fixed and increasing the speed by a factor of 4.
C) keeping the radius fixed and increasing the period by a factor of 4.
D) keeping the radius fixed and decreasing the period by a factor of 4.
E) keeping the speed fixed and decreasing the radius by a factor of 4.