12. A proton with an initial kinetic energy of 9.50 eV is fired directly at another proton whose location is fixed. When the moving proton has reached its point of closest approach, by how much has the electric potential energy of this two-particle system changed:

(a) +9.50 eV,

(b) −9.50 eV,

(c) zero, or

(d) it depends on the distance of closest approach, so you can’t tell from the data given?