8. 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