A marble is placed at the bottom of a semi-spherical bowl, as shown in the figure. The marble is then displaced from the bottom of the bowl to a position about halfway from the top of the bowl. The marble is then released from rest such that the marble always remains in contact with the bowl. Students observe that the marble rolls back and forth as it oscillates about the bottom of the bowl. Which of the following statements best explains why the marble undergoes oscillatory motion?

A marble is placed at the bottom of a semi-spherical bowl, as shown in the figure. The marble is then displaced from the bottom of the bowl to a position about halfway from the top of the bowl. The marble is then released from rest such that the marble always remains in contact with the bowl. Students observe that the marble rolls back and forth as it oscillates about the bottom of the bowl. Which of the following statements best explains why the marble undergoes oscillatory motion?