Suppose We Use a String to Suspend a Ball from a Hook

Multiple Choice

Suppose we use a string to suspend a ball from a hook in the ceiling. As the ball swings back and forth, which interaction does work on the system if
-we consider the system to be the ball and the earth?

A) The ball's gravitational interaction with the earth
B) The ball's contact interaction with the string
C) Both interactions
D) Neither interaction