You accidentally bump into a bookcase. It tilts briefly but then returns to upright, and you breath a sigh of relief. When you first bumped the bookcase, its center of gravity moved:

Explanation

The correct answer is that the center of gravity moved upward and its gravitational potential energy increased. This increase in potential energy indicates that the bookcase is in a stable equilibrium, as it returns to its upright position after being displaced. This is consistent with the characteristics of stable equilibrium, where a displacement leads to an increase in potential energy, prompting the system to revert to its equilibrium state. The other options are incorrect as they do not align with the principles of stable equilibrium and the observed behavior of the bookcase.