A tether ball is attached to the top of a vertical pole by a string. The tether ball moves in a circle in the horizontal plane. The string sweeps out the surface of a right circular cone, with the apex of the cone at the top where the string is attached to the top of the pole. Which of the following causes the tether ball to move with uniform circular motion? a. Gravitational force b. Tension in the string c. Vertical component of the tension in the string d. Horizontal component of the tension in the string