Dr. Henderson teaches astronomy. She used to keep a model of Mars on the edge of her desk, but one day she decided to hang it up so that her students could see it better. She used a string to tie the model to a hook in the ceiling. A few days later, the string came loose and the model fell to the ground. When was the most gravitational potential energy stored between the model and Earth? Assume that the model's mass did not change.

A. when the model was on the edge of Dr. Henderson's desk
B. when the model was hanging from the hook
C. when the model was about to hit the ground