Two identical solid spheres are initially separated by a center-to-center distance r0. The spheres are then slowly moved until the separation is doubled to 2r0. Which statement best describes how the gravitational potential energy of the two-sphere system changes?
a) It becomes less negative, and its magnitude doubles.
b) It becomes more negative, and its magnitude is reduced by a factor of 21.
c) It becomes less negative, and its magnitude is reduced by a factor of 2.
d) It becomes less negative, and its magnitude is reduced by a factor of 4.