A −4.0 nC point charge and a +6.0 nC point charge are separated by 2.5 cm inside a dielectric material of relative permittivity ε_r = 2. What is the electric potential energy U of the pair? (Use k ≈ 9.0 × 10⁹ N·m²/C².)
a) +4.3μJ
b) −2.2μJ
c) −4.3μJ
d) −8.6μJ