A ball moves in a circle at the end of a string, as shown.

An illustration of a circle with an arrowhead on the circle pointing clockwise. A black dot on the top left part of the circle is a dot with 4 vectors from it. Vector Z away from the center of the circle, vector Y toward the center of the circle, a vector tangent to the circle and clockwise labeled W and a vector labeled X along the circle and in the clockwise direction.
Which arrow represents the centripetal force responsible for this motion?

W
X
Y
Z