A figure skater is spinning on nearly frictionless ice with no external torques acting on the system. The skater pulls her arms in toward her rotation axis. Which of the following best describes how her rotational kinetic energy changes?
a) It stays the same because, in the absence of external torques, mechanical energy is conserved.
b) It increases because her moment of inertia decreases while her angular momentum remains constant.
c) It decreases because pulling her arms inward does negative work on the system, removing rotational energy.
d) It decreases because her angular momentum becomes smaller when the radius of rotation is reduced.